Network Operating System

Network Operating System

Features..



Network Support


A network operating system must support a wide variety of networking protocols in order to meet the needs of its users. That’s because a large network typically consists of a mixture of various versions of Windows, as well as Macintosh and possibly Linux computers. As a result, the server may need to simultaneously support TCP/IP, NetBIOS, and AppleTalk protocols. Many servers have more than one network interface card installed. In that case, the NOS must be able to support multiple network connections. Ideally, the NOS should have the ability to balance the network load among its network interfaces. In addition, in the event that one of the connections fails, the NOS should be able to seamlessly switch to the other connection. Finally, most network operating systems include a built-in ability to function as a router that connects two networks. The NOS router functions should also include firewall features in order to keep unauthorized packets from
entering the local network.


File sharing services
 
One of the most important functions of a network operating system is its ability to share resources with other network users. The most common resource that’s shared is the server’s file system. A network server must be able to share some or all its disk space with other users so that those users can treat the server’s disk space as an extension of their own computer’s disk space. The NOS allows the system administrator to determine which portions of the server’s file system to share. Although an entire hard drive can be shared, it is not commonly done. Instead, individual directories or folders are shared. The administrator can control which users are allowed to access each shared folder. Because file sharing is the reason many network servers exist, network operating systems have more sophisticated disk management features than are found in desktop operating systems. For example, most network operating systems have the ability to manage two or more hard drives as if they were a single drive. In addition, most can create mirrors, which automatically keeps a backup copy of a drive on a second drive.


Multitasking
 
Only one user at a time uses a desktop computer; however, multiple users simultaneously use server computers . As a result, a network operating system must provide support for multiple users who access the server remotely via the network.At the heart of multiuser support is multitasking, which is the ability of an operating system to execute more than one program — called a task or a process — at a time. Although multitasking creates the appearance that two or more programs are executing on the computer at one time, in reality, a computer with a single processor can execute only one program at a time. The operating system switches the CPU from one program to another to create the appearance that several programs are executing simultaneously, but at any given moment, only one of the programs is actually executing. The others are patiently waiting for their turns. (If the computer has more than one CPU, the CPUs can execute
programs simultaneously, which is called multiprocessing.) To see multitasking in operating on a Windows computer, press Ctrl+Alt+Delete to bring up the Windows Task Manager and then click the Processes tab. All the tasks currently active on the computer appear. In order for multitasking to work reliably, the network operating system must completely isolate the executing programs from each other. Otherwise, one program may perform an operation that adversely affects another program. Multitasking operating systems do this by providing each task with its own unique address space that makes it almost impossible for one task to affect memory that belongs to another task.


In most cases, each program executes as a single task or process within the memory address space allocated to the task. However, a single program can also be split into several tasks. This technique is usually called multithreading, and the program’s tasks are called threads. The two approaches to multitasking are preemptive and non-preemptive. In preemptive multitasking, the operating system decides how long each task gets to execute before it should step aside so that another task can execute. When a task’s time is up, the operating system’s task manager interrupts the task and switches to the next task in line. All the network operating systems in widespread use today use preemptive multitasking.
The alternative to preemptive multitasking is non-preemptive multitasking. In non-preemptive multitasking, each task that gets control of the CPU is allowed to run until it voluntarily gives up control so that another task can run. Non-preemptive multitasking requires less operating system overhead because he operating system doesn’t have to keep track of how long each task has run. However, programs have to be carefully written so that they don’t hog the computer all to themselves.




Directory services

Directories are everywhere. When you need to make a phone call, you look up the number in a phone directory. When you need to find the address of a client, you look up him or her in your Rolodex. And when you need to find the Sam Goody store at a shopping mall, you look for the mall directory. Networks have directories, too. Network directories provide information about the resources that are available on the network, such as users, computers, printers, shared folders, and files. Directories are an essential part of any network operating system. In early network operating systems, such as Windows NT 3.1 and NetWare 3.x, each server computer maintained its own directory database of resources that were available just on that server. The problem with that approach was that network administrators had to maintain each directory database separately. That wasn’t too bad for networks with just a few servers, but maintaining the directory on a network with dozens or even hundreds of servers was next to impossible. In addition, early directory services were application-specific. For example, a server would have one directory database for user logins, another for file sharing, and yet another for e-mail addresses. Each directory had its own tools for adding, updating, and deleting directory entries. 

Modern network operating systems provide global directory services that combine the directory information for an entire network and for all applications so that it can be treated as a single integrated database. These directory services are based on an ISO standard called X.500. In an X.500 directory, information is organized hierarchically. For example, a multinational company can divide its user directory into one or more countries, each country can have one or more regions, and, in turn, each region can have one or more departments.


Security services
 
All network operating systems must provide some measure of security to protect the network from unauthorized access. Hacking seems to be the national pastime these days. With most computer networks connected to the Internet, anyone anywhere in the world can and probably will try to break into your network. The most basic type of security is handled through user accounts, which grant individual users the right to access the network resources and govern what resources the user can access. User accounts are secured by passwords; therefore, good password policy is a cornerstone of any security system. Most network operating systems let you establish password policies, such as requiring that passwords have a minimum length and include a mix of letters and numerals. In addition, passwords can be set to expire after a certain number of days, so users can be forced to frequently change their passwords.
Most network operating systems also provide for data encryption, which scrambles data before it is sent over the network or saved on disk, and digital certificates, which are used to ensure that users are who they say they are and files are what they claim to be.

Network Attached Storage

Network Attached Storage

Many network servers exist solely for the purpose of making disk space available to network users. As networks grow to support more users, and users require more disk space, network administrators are constantly finding ways to add more storage to their networks. One way to do that is to add additional file servers. However, a simpler and less expensive way is to use network attached storage, also known as NAS.
A NAS device is a self-contained file server that’s preconfigured and ready to run. All you have to do to set it up is take it out of the box, plug it in, and turn it on. NAS devices are easy to set up and configure, easy to maintain, and less expensive than traditional file servers. NAS should not be confused with a related technology called storage area networks, or SAN. SAN is a much more complicated and expensive technology
that provides huge quantities of data storage for large networks.




Note that some NAS devices use customized versions of Linux rather than the Windows 2000 Server Appliance Kit. Also, in some systems, the operating system resides on a separate hard drive that’s isolated from the shared disks. This prevents the user from inadvertently damaging the operating system.

Routers

Routers


A router is like a bridge, but with a key difference. Bridges are Data Link layer devices, so they can tell the MAC address of the network node to which each message is sent, and can forward the message to the appropriate segment. However, they can’t peek into the message itself to see what type of information is being sent. In contrast, a router is a Network layer device, so it can work with the network packets at a higher level. In particular, a router can examine the IP address of the packets that pass through it. And because IP addresses have both a network and a host address, a router can determine what network a message is coming from and going to. Bridges are ignorant of this information.

One key difference between a bridge and a router is that a bridge is essentially transparent to the network. In contrast, a router is itself a node on the network, with its own MAC and IP addresses. This means that messages can be directed to a router, which can then examine the contents of the message to determine how it should handle the message. You can configure a network with several routers that can work cooperatively
together. For example, some routers are able to monitor the network to determine the most efficient path for sending a message to its ultimate destination. If a part of the network is extremely busy, a router can automatically route messages along a less-busy route. In this respect, the router is kind of like a traffic reporter up in a helicopter. The router knows that 101 is bumper-tobumper all the way through Sunnyvale, so it sends the message on 280 instead.


✦ Routers aren’t cheap. For big networks, though, they’re worth it.

✦ The functional distinctions between bridges and routers — and
switches and hubs, for that matter — get blurrier all the time. As
bridges, hubs, and switches become more sophisticated, they’re able to take on some of the chores that used to require a router, thus putting many routers out of work.

✦ Some routers are nothing more than computers with several network interface cards and special software to perform the router functions.

✦ Routers can also connect networks that are geographically distant from each other via a phone line (using modems) or ISDN.

✦ You can also use a router to join your LAN to the Internet.

Repeaters and Bridges

 Repeaters and Bridges


A repeater is a gizmo that gives your network signals a boost so that the signals can travel farther. It’s kind of like the Gatorade stations in a marathon. As the signals travel past the repeater, they pick up a cup of  Gatorade, take a sip, splash the rest of it on their heads, toss the cup, and hop in a cab when they’re sure that no one is looking. You need a repeater when the total length of a single span of network cable is larger than the maximum allowed for your cable type: 
Cable                                                      Maximum Length
10Base2  (Coaxial)                                 185 meters or 606 feet
10/100BaseT (Twisted Pair)                   100 meters or 328 feet


Repeaters are used only with Ethernet networks wired with coaxial cable; 10/100BaseT networks don’t use repeaters.

Bridges

A bridge is a device that connects two networks so that they act as if they are one network. Bridges are used to partition one large network into two smaller networks for performance reasons. You can think of a bridge as a kind of smart repeater. Repeaters listen to signals coming down one network cable, amplify them, and send them down the other cable. They do this blindly, paying no attention to the content of the messages that they repeat. In contrast, a bridge is a little smarter about the messages that come down the pike. For starters, most bridges have the capability to listen to the network and automatically figure out the address of each computer on both sides of the bridge. Then the bridge can inspect each message that comes from one side of the bridge and broadcast it on the other side of the bridge, but only if the message is intended for a computer that’s on the other side. This key feature enables bridges to partition a large network into two smaller, more efficient networks. Bridges work best in networks that are highly segregated.


A bridge can partition the Sneetchnet into two networks: the Star-Bellied network and the Plain-Bellied network. The bridge automatically learns which computers are on the Star-Bellied network and which are on the Plain-Bellied network. The bridge forwards messages from the Star-Bellied side to the Plain-Bellied side (and vice versa) only when necessary. The overall performance of both networks improves, although the performance of any network operation that has to travel over the bridge slows down a bit.

Hubs and Switches

 Hubs and Switches

The biggest difference between using coaxial cable and twisted-pair cable is that when you use twisted-pair cable, you also must use a separate device called a hub. With twisted-pair cabling, you can more easily add new computers to the network, move computers, find and correct cable problems, and service the computers that you need to remove from the network temporarily.  

A switch is simply a more sophisticated type of hub.


If you use twisted-pair cabling, you need to know some of the ins and outs of using hubs:
 
✦ Because you must run a cable from each computer to the hub or switch, find a central location for the hub or switch to which you can easily route the cables.

✦ The hub or switch requires electrical power, so make sure that an electrical outlet is handy.

✦ When you purchase a hub or switch, purchase one with at least twice as many connections as you need. Don’t buy a four-port hub or switch if you want to network four computers because when (not if) you add the fifth computer, you have to buy another hub or switch.

✦ You can connect hubs or switches to one another, as shown in Figure 3-3; this is called daisy-chaining. When you daisy-chain hubs or switches, you connect a cable to a standard port on one of the hubs or  switches and the daisy-chain port on the other hub or switch. Be sure to read the instructions that come with the hub or switch to make sure that you daisy-chain them properly.

✦ You can daisy-chain no more than three hubs or switches together. If you have more computers than three hubs can accommodate, don’t panic. For a small additional cost, you can purchase hubs that have a BNC connection on the back. Then you can string the hubs together using thinnet cable. The three-hub limit doesn’t apply when you use thinnet cable to connect the hubs. You can also get stackable hubs or switches that have high-speed direct connections that enable two or more hubs or switches to be counted as a single hub or switch.

Network Cable

Network Cable


You can construct an Ethernet network by using one of two different types of cable: coaxial cable, which resembles TV cable, or twisted-pair cable, which looks like phone cable. Twisted-pair cable is sometimes called UTP, or 10BaseT cable, for reasons I try hard not to explain later (in the section “Twisted-pair cable”). 

You may encounter other types of cable in an existing network: thick yellow cable that used to be the only type of cable used for Ethernet, fiber-optic cables that span long distances at high speeds, or thick twisted-pair bundles that carry multiple sets of twisted-pair cable between wiring closets in a large building. For all but the largest networks, the choice is between coaxial cable and twisted-pair cable.



Coaxial cableOne type of cable that you can use for Ethernet networks is coaxial cable, usually called thinnet or sometimes BNC cable because of the type of connectors used on each end of the cable. Thinnet cable operates only at 10Mbps and is rarely used for new networks. However, you’ll find plenty of existing thinnet networks still being used.


Here are some salient points about coaxial cable:

✦ You attach thinnet to the network interface card by using a goofy twiston connector called a BNC connector. You can purchase pre-assembled cables with BNC connectors already attached in lengths of 25 or 50 feet, or you can buy bulk cable on a big spool and attach the connectors yourself by using a special tool. (I suggest buying pre-assembled cables. Attaching connectors to bulk cable can be tricky.)
 
✦ With coaxial cables, you connect your computers point-to-point in a bus topology. At each computer, a T connector is used to connect two cables to the network interface card.
 
✦ A special plug called a terminator is required at each end of a series of thinnet cables. The terminator prevents data from spilling out the end of the cable and staining the carpet.
 
✦ The cables strung end-to-end from one terminator to the other are collectively called a segment. The maximum length of a thinnet segment is about 200 meters (actually, 185 meters). You can connect as many as 30 computers on one segment. To span a distance greater than 185 meters or to connect more than 30 computers, you must use two or more segments with a device called a repeater to connect each segment.
Although Ethernet coaxial cable resembles TV coaxial cable, the two types of cable are not interchangeable. Don’t try to cut costs by wiring your network with cheap TV cable.




Twisted-pair cable
A popular alternative to thinnet cable is twisted-pair cable, or UTP. (The U stands for unshielded, but no one says unshielded twisted pair. Just twisted pair will do.) UTP cable is even cheaper than thin coaxial cable, and best of all, many modern buildings are already wired with twisted-pair cable because this type of wiring is often used with modern phone systems. 
When you use UTP cable to construct an Ethernet network, you connect the computers in a star arrangement. In the center of the star is a device called a hub. Depending on the model, Ethernet hubs enable you to connect from 4 to 24 computers using twisted-pair cable. An advantage of UTP’s star arrangement is that if one cable goes bad, only the computer attached to that cable is affected; the rest of the network continues to chug along. With coaxial cable, a bad cable affects the entire network, and not just the computer to which the bad cable is connected.

Here are a few other details that you should know about twisted-pair cabling:
 
✦ UTP cable consists of pairs of thin wire twisted around each other; several such pairs are gathered up inside an outer insulating jacket. Ethernet uses two pairs of wires, or four wires altogether. The number of pairs in a UTP cable varies, but it is often more than two. 

✦ UTP cable comes in various grades called Categories. Don’t use anything less than Category 5 cable for your network. Although cheaper, it may not be able to support faster networks. Although higher Category cables are more expensive than lower Category cables, the real cost of installing Ethernet cabling is the labor required to actually pull the cables through the walls. As a result, I recommend that you always spend the extra money to buy Category 5 cable

✦ The maximum allowable cable length between the hub and the computer is 100 meters (about 328 feet).

Network Interface Cards

Network Interface Cards

Every computer on a network, both clients and servers, requires a network interface card (or NIC) in order to access the network. A NIC is usually a separate adapter card that slides into one of the server’s motherboard expansion slots. However, some motherboards have a built-in network interface,
so a separate card isn’t needed. 

✦ A NIC is a Physical layer and Data Link layer device. Because a NIC establishes a network node, it must have a physical network address, also known as a MAC address. The MAC address is burned into the NIC at the factory, so you can’t change it. Every NIC ever manufactured has a unique MAC address.


✦ For server computers, it makes sense to use more than one NIC. That way, the server can handle more network traffic. Some server NICs have two or more network interfaces built into a single card.

✦ Fiber-optic networks also require NICs. Fiber-optic NICs are still too expensive for desktop use in most networks. Instead, they’re used for high-speed backbones. If a server connects to a high-speed fiber backbone, it will need a fiber-optic NIC that matches the fiber-optic cable being used.

✦ Long ago and far away, Novell manufactured a network interface card known as the NE2000. The NE2000 card is no longer made, but NE2000 remains a standard of compatibility for network interface cards. If a card is NE2000-compatible, you can use it with just about any network. If you buy a card that is not NE2000-compatible, make sure that the card is compatible with the network operating system that you intend to use.

Servers

Servers

Server computers are the lifeblood of any network. Servers provide the shared resources that network users crave, such as file storage, databases, e-mail, Web services, and so on. Choosing the equipment you use for your network’s servers is one of the key decisions you’ll make when you set up a network. In this section, I describe some of the various ways you can equip your network’s servers.


What’s important in a server

✦ Scalability: Scalability refers to the ability to increase the size and capacity of the server computer without unreasonable hassle. It is a major mistake to purchase a server computer that just meets your current needs because, you can rest assured, your needs will double within a year. If at all possible, equip your servers with far more disk space, RAM, and processor power than you currently need.

✦ Reliability: The old adage “you get what you pay for” applies especially well to server computers. Why spend $3,000 on a server computer when you can buy one with similar specifications at a discount electronics store for $1,000? One reason is reliability. When a client computer fails, only the person who uses that computer is affected. When a server fails, however, everyone on the network is affected. The less expensive computer is probably made of inferior components that are more likely to fail.
 
✦ Availability: This concept of availability is closely related to reliability. When a server computer fails, how long does it take to correct the problem and get the server up and running again? Server computers are designed so that their components can be easily diagnosed and replaced, thus minimizing the downtime that results when a component fails. In some servers, components are hot swappable, which means that certain components can be replaced without shutting down the server. Some servers are designed to be fault-tolerant so that they can continue to operate even if a major component fails. For more information about fault-tolerant computers, see the sidebar “Tolerant to a fault.”

✦ Service and support: Service and support are factors often overlooked when picking computers. If a component in a server computer fails, do you have someone on site qualified to repair the broken computer? If not, you should get an on-site maintenance contract for the computer. Don’t settle for a maintenance contract that requires you to take the computer in to a repair shop or, worse, mail it to a repair facility. You
can’t afford to be without your server that long. 

Components of a server computer


✦ Motherboard: The motherboard is a single, large electronic circuit board to which all the other components of your computer are connected. More than any other component, the motherboard is the computer. All other components attach to the motherboard. The major components on the motherboard include the processor (or CPU), supporting circuitry called the chipset, memory, expansion slots, a standard IDE hard drive controller, and I/O ports for devices such as keyboards, mice, and printers. Some motherboards also include additional built-in features such as a graphic adapter, SCSI disk controller, or
a network interface.

✦ Processor: The processor, or CPU, is the brain of the computer. Although the processor isn’t the only component that affects overall system performance, it is the one that most people think of first when deciding what type of server to purchase. At the time of this writing, Intel had four processor models, summarized in Table 3-1. Two of them — the Pentium 4 and Celeron — should be used only for desktop or notebook computers. Server computers should have an Itanium 2 or a Xeon processor, or a comparable processor from one of Intel’s competitors, such as AMD. Each motherboard is designed to support a particular type of processor. CPUs come in two basic mounting styles: slot or socket. However, you can choose from several types of slots and sockets, so you have to make sure that the motherboard supports the specific slot or socket style used by the CPU. Some server motherboards have two or more
slots or sockets to hold two or more CPUs. The term clock speed refers to how fast the basic clock that drives the processor’s operation ticks. In theory, the faster the clock speed, the faster the processor. However, clock speed alone is reliable only for comparing processors within the same family. In fact, the Xeon is significantly faster than the Pentium 4 running at the same clock speed. That’s because the Xeon contains more advanced circuitry than the Pentium 4, so it’s able to accomplish more work than the Pentium 4 with each tick of the clock. 

✦ Memory: Don’t scrimp on memory. People rarely complain about servers having too much memory. Many different types of memory are available, so you have to pick the right type of memory to match the memory supported by your motherboard. The total memory capacity of the server depends on the motherboard. Most new servers can support up to 12GB of memory.

✦ Hard drives: Most desktop computers use inexpensive hard drives called IDE drives. These drives are adequate for individual users, but because performance is more important for servers, another type of drive known as SCSI is usually used instead. For the best performance, use the SCSI drives along with a high-performance SCSI controller card. 

✦ Network connection: The network connection is one of the most important parts of any server. Many servers have network adapters built into the motherboard. If your server isn’t equipped as such, you’ll need to add a separate network adapter card. See the section “Network Adapters,” later in this chapter, for more information. 

✦ Video: Fancy graphics aren’t that important for a server computer. You can equip your servers with inexpensive generic video cards and monitors without affecting network performance. (This is one of the few areas where it’s acceptable to cut costs on a server.) 

✦ Power supply: Because a server usually has more devices than a typical desktop computer, it requires a larger power supply (300 watts is typical). If the server houses a large number of hard drives, it may require an even larger power supply.


Server form factors


 The term form factor refers to the size, shape, and packaging of a hardware device. Server computers typically come in one of three form factors:

✦ Tower case: Most servers are housed in a traditional tower case, similar to the tower cases used for desktop computers. A typical server tower case is 18 inches high, 20 inches deep, and 9 inches wide and has room inside for a motherboard, five or more hard drives, and other components. Tower cases also come with built-in power supplies. Some server cases include advanced features specially designed for servers, such as redundant power supplies (so both servers can continue operating if one of the power supplies fails), hot-swappable fans, and hot-swappable disk drive bays. (Hot-swappable components can be replaced without powering down the server.)
 
✦ Rack mount: If you only need a few servers, tower cases are fine. You can just place the servers next to each other on a table or in a cabinet that’s specially designed to hold servers. If you need more than a few servers, though, space can quickly become an issue. For example, what if your departmental network requires a bank of ten file servers? You’d need a pretty long table. Rack-mount servers are designed to save space when you need more than a few servers in a confined area. A rack-mount server is housed in a small chassis that’s designed to fit into a standard 19-inch equipment rack. The rack allows you to vertically stack servers in order to save space. Because of their small size, rack-mount servers are not as expandable as tower-style servers. A typical system includes built-in video and network connections, room for three hard drives, two empty expansion slots for additional adapters, and a SCSI port to connect additional external hard drives.
 
✦ Blade servers: Blade servers are designed to save even more space than rack-mount servers. A blade server is a server on a single card that can be mounted alongside other blade servers in a blade chassis, which itself fits into a standard 19-inch equipment rack. A typical blade chassis holds six or more servers, depending on the manufacturer. One of the key benefits of blade servers is that you don’t need a separate power supply for each server. Instead, the blade enclosure provides power for all its blade servers. Some blade server systems provide rack-mounted power supplies that can serve several blade enclosures mounted in a single rack. In addition, the blade enclosure provides KVM switching so that you don’t have to use a separate KVM switch. You can control any of the servers in a blade server network from a single keyboard, monitor, and mouse. One of the biggest benefits of blade servers is that they drastically cut down the amount of cable clutter. With rack-mount servers, each server requires its own power cable, keyboard cable, video cable, mouse cable, and network cables. With blade servers, a single set of cables can service all the servers in a blade enclosure.

Other Protocols

Other Protocols Worth Knowing About


Other networks besides Ethernet, TCP/IP, and IPX/SPX are worth knowing about:
 
✦ NetBIOS: Short for Network Basic Input Output System, this is the basic application-programming interface for network services on Windows computers. It is installed automatically when you install TCP/IP, but doesn’t show up as a separate protocol when you view the network connection properties (refer to Figure 2-1). NetBIOS is a Session layer protocol that can work with Transport layer protocols such as TCP, SPX, or
NetBEUI.

✦ NetBEUI: Short for Network BIOS Extended User Interface, this is a Transport layer protocol that was designed for early IBM and Microsoft networks. NetBEUI is now considered obsolete. 

✦ AppleTalk: Apple computers have their own suite of network protocols known as AppleTalk. The AppleTalk suite includes a Physical and Data Link layer protocol called LocalTalk, but can also work with standard lower level protocols, including Ethernet and Token Ring.

✦ SNA: Systems Network Architecture is an IBM networking architecture that dates back to the 1970s, when mainframe computers roamed the earth and PCs had barely emerged from the primordial computer soup.
SNA was designed primarily to support huge terminals such as airline reservation and banking systems, with tens of thousands of terminals attached to central host computers. Now that IBM mainframes support TCP/IP and terminal systems have all but vanished, SNA is beginning to fade away. Still, many networks that incorporate mainframe computers have to contend with SNA.

The TCP/IP Protocol Suite

The TCP/IP Protocol Suite

TCP/IP, the protocol on which the Internet is built, is actually not a single protocol but rather an entire suite of related protocols. TCP is even older than Ethernet. It was first conceived in 1969 by the Department of Defense. For more on the history of TCP/IP, see the sidebar “The fascinating story of TCP/IP,” later in this chapter. Currently, the Internet Engineering Task Force, or IETF, manages the TCP/IP protocol suite.
 
The TCP/IP suite is based on a four-layer model of networking that is similar to the seven-layer OSI model. Figure 2-7 shows how the TCP/IP model matches up with the OSI model and where some of the key TCP/IP protocols fit into the model. As you can see, the lowest layer of the model, the Network Interface layer, corresponds to the OSI model’s Physical and Data Link layers. TCP/IP can run over a wide variety of Network Interface layer protocols, including Ethernet, as well as other protocols, such as Token Ring
and FDDI (an older standard for fiber-optic networks).

The Application layer of the TCP/IP model corresponds to the upper three layers of the OSI model — that is, the Session, Presentation, and Application layers. Many protocols can be used at this level. A few of the most popular are HTTP, FTP, Telnet, SMTP, DNS, and SNMP.


IP
IP, which stands for Internet Protocol, is a Network layer protocol that is responsible for delivering packets to network devices. The IP protocol uses logical IP addresses to refer to individual devices rather than physical (MAC) addresses. A protocol called ARP (for Address Resolution Protocol) handles the task of converting IP addresses to MAC addresses. Because IP addresses consist of a network part and a host part, IP is a routable protocol. As a result, IP can forward a packet to another network if the host is not on the current network. (The ability to route packets across networks is where IP gets its name. An internet is a series of two or more connected TCP/IP networks that can be reached by routing.)


TCP
TCP, which stands for Transmission Control Protocol, is a connection-oriented Transport layer protocol. TCP lets a device reliably send a packet to another device on the same network or on a different network. TCP ensures that each packet is delivered if at all possible. It does so by establishing a connection with the receiving device and then sending the packets. If a packet doesn’t arrive, TCP resends the packet. The connection is closed only after the packet has been successfully delivered or an unrecoverable error condition has occurred. One key aspect of TCP is that it’s always used for one-to-one communications.
In other words, TCP allows a single network device to exchange data with another single network device. TCP is not used to broadcast messages to multiple network recipients. Instead, the User Datagram Protocol (UDP) is used for that purpose. Many well-known Application layer protocols rely on TCP. For example,
when a user running a Web browser requests a page, the browser uses HTTP to send a request via TCP to the Web server. When the Web server receives the request, it uses HTTP to send the requested Web page back to the browser, again via TCP. Other Application layer protocols that use TCP include Telnet (for terminal emulation), FTP (for file exchange), and SMTP (for e-mail).


UDP
The User Datagram Protocol (or UDP) is a connectionless Transport layer protocol that is used when the overhead of a connection is not required. After UDP has placed a packet on the network (via the IP protocol), it forgets about it. UDP doesn’t guarantee that the packet actually arrives at its destination. Most applications that use UDP simply wait for any replies expected as a result of packets sent via UDP. If a reply doesn’t arrive within a certain period of time, the application either sends the packet again or gives up.
Probably the best known Application layer protocol that uses UDP is DNS, the Domain Name System. When an application needs to access a domain name such as www.wiley.com, DNS sends a UDP packet to a DNS server to look up the domain. When the server finds the domain, it returns the domain’s IP address in another UDP packet.


The IPX/SPX Protocol Suite
Although TCP/IP has quickly become the protocol of choice for most networks, plenty of networks still use an alternative protocol suite called IPX/SPX. Novell originally developed the IPX/SPX suite in the 1980s for use with their NetWare servers. IPX/SPX also works with all Microsoft operating systems, with OS/2, and even with UNIX and Linux. NetWare versions 5.0 and later fully support TCP/IP, so you don’t have to
use IPX/SPX with Novell networks unless the network has a server that runs NetWare 4.x or (heaven forbid) 3.x. If your network doesn’t have one of the older NetWare servers, you’re better off using TCP/IP instead of IPX/SPX. Here are a few other points to know about IPX/SPX: 
✦ IPX stands for Internetwork Package Exchange. It’s a Network layer protocol that’s analogous to IP.
✦ SPX stands for Sequenced Package Exchange. It’s a Transport layer protocol that’s analogous to TCP.
✦ Unlike TCP/IP, IPX/SPX is not a standard protocol established by a standards group, such as IEEE. Instead, IPX/SPX is a proprietary standard developed and owned by Novell. Both IPX and IPX/SPX are registered trademarks of Novell, which is why Microsoft’s versions of IPX/SPX aren’t called simply “IPX/SPX.”

The Ethernet Protocol

The Ethernet Protocol

As you know, the first two layers of the OSI model deal with the physical structure of the network and the means by which network devices can send information from one device on a network to another. By far, the most popular set of protocols for the Physical and Data Link layers is Ethernet.



Ethernet has been around in various forms since the early 1970s. (For a brief history of Ethernet, see the sidebar “Ethernet folklore and mythology,” later in this chapter.) The current incarnation of Ethernet is defined by the IEEE standard known as 802.3. Various flavors of Ethernet operate at different speeds and use different types of media. However, all the versions of Ethernet are compatible with each other, so you can mix and match them on the same network by using devices such as bridges, hubs, and switches to link network segments that use different types of media. The actual transmission speed of Ethernet is measured in millions of bits per second, or Mbps. Ethernet comes in three different speed versions: 10Mbps, known as Standard Ethernet; 100Mbps, known as Fast Ethernet; and 1000Mbps, known as Gigabit Ethernet. Keep in mind, however, that network transmission speed refers to the maximum speed that can be achieved over the network under ideal conditions. In reality, the actual throughput of an Ethernet network rarely reaches this maximum speed. Ethernet operates at the first two layers of the OSI model — the Physical and the Data Link layers. However, Ethernet divides the Data Link layer into two separate layers known as the Logical Link Control (LLC) layer and the Medium Access Control (MAC) layer.




 


Standard EthernetStandard Ethernet is the original Ethernet. It runs at 10Mbps, which was considered fast in the 1970s but is pretty slow by today’s standards. Because the cost of Fast Ethernet has dropped dramatically in the past few years, Fast Ethernet has pretty much replaced Standard Ethernet for most new networks. However, plenty of existing Standard Ethernet networks are still in use. Standard Ethernet comes in four incarnations, depending on the type of cable used to string the network together:
✦ 10Base5: The original Ethernet cable was thick (about as thick as your thumb), heavy, and difficult to work with. It is seen today only in museums.
✦ 10Base2: This thinner type of coaxial cable (it resembles television cable) became popular in the 1980s and lingered into the early 1990s. Plenty of 10Base2 cable is still in use, but it’s rarely installed in new networks. 10Base2 (like 10Base5) uses a bus topology, so wiring a 10Base2 network involves running cable from one computer to the next until all the computers are connected in a segment.
✦ 10BaseT: Unshielded twisted-pair cable (also known as UTP) became popular in the 1990s because it’s easier to install, lighter, more reliable, and offers more flexibility in how networks are designed. 10BaseT networks use a star topology with hubs at the center of each star. Although the maximum length of 10BaseT cable is only 100 meters, hubs can be chained together to extend networks well beyond the 100 meter limit.
10BaseT cable has four pairs of wires that are twisted together throughout the entire span of the cable. However, 10BaseT uses only two of these wire pairs, so the unused pairs are spares.
✦ 10BaseFL: Fiber-optic cables were originally supported at 10Mbps by the 10BaseFL standard.  However, because faster fiber-optic versions of Ethernet now exist, 10BaseFL is rarely used.


Fast EthernetFast Ethernet refers to Ethernet that runs at 100Mbps, which is ten times the speed of standard Ethernet. The following are the three varieties of fast Ethernet:
✦ 100BaseT4: The 100BaseT4 protocol allows transmission speeds of 100Mbps over the same UTP cable as 10BaseT networks. To do this, it uses all four pairs of wire in the cable. 100BaseT4 simplifies the task of
upgrading an existing 10BaseT network to 100Mbps.
✦ 100BaseTX: The most commonly used standard for office networks today is 100BaseTX, which transmits at 100Mbps over just two pairs of a higher grade of UTP cable than the cable used by 10BaseT. The highergrade cable is referred to as Category 5. Most new networks are wired with Category 5 or better cable.
✦ 100BaseFX: The fiber-optic version of Ethernet running at 100Mbps is called 10BaseFX. Because fiber-optic cable is expensive and tricky to install, it isn’t used much for individual computers in a network.
However, it’s commonly used as a network backbone. For example, a fiber backbone is often used to connect individual workgroup hubs to routers and servers.


Gigabit EthernetGigabit Ethernet is Ethernet running at a whopping 1,000Mbps, which is 100 times faster than the original 10Mbps Ethernet. Gigabit Ethernet is considerably more expensive than Fast Ethernet, so it’s typically used only when the improved performance justifies the extra cost. For example, you may find Gigabit Ethernet used as the backbone for very large networks or to connect server computers to the network. And in some cases, Gigabit Ethernet is even used for desktop computers that require high-speed network connections.
Gigabit Ethernet comes in two flavors:
✦ 1000BaseT: Gigabit Ethernet can run on Category 5 UTP cable, but higher grades such as Category 5e or Category 6 is preferred because it is more reliable.
✦ 1000BaseLX: Several varieties of fiber cable are used with Gigabit Ethernet, but the most popular is called 1000BaseLX.

The Seven Layers of the OSI Reference Model

The Seven Layers of the OSI Reference Model

OSI stands for in the networking world is Open Systems Interconnection, as in the Open Systems Interconnection Reference Model, affectionately known as the OSI model. The OSI model breaks the various aspects of a computer network into seven distinct layers. These layers are kind of like the layers of an onion: Each successive layer envelops the layer beneath it, hiding its details from the levels above. The OSI model is also like an onion in that if you start to peel it apart to have a look inside, you’re bound to shed a few tears.
The OSI model is not a networking standard in the same sense that Ethernet and Token Ring are networking standards. Rather, the OSI model is a framework into which the various networking standards can fit. The OSI model specifies what aspects of a network’s operation can be addressed by various network standards. So, in a sense, the OSI model is sort of a standard of standards.




The Physical Layer
The bottom layer of the OSI model is the Physical layer. It addresses the physical characteristics of the network, such as the types of cables used to connect devices, the types of connectors used, how long the cables can be, and so on. Another aspect of the Physical layer is the electrical characteristics of the
signals used to transmit data over the cables from one network node to another. The Physical layer doesn’t define any meaning to those signals other than the basic binary values of zero and one. The higher levels of the OSI model must assign meanings to the bits that are transmitted at the Physical layer. One type of Physical layer device commonly used in networks is a repeater. A repeater is used to regenerate the signal whenever you need to exceed the cable length allowed by the Physical layer standard. 10BaseT hubs are also Physical layer devices. Technically, they’re known as multiport repeaters because the purpose of a hub is to regenerate every packet received on any port on all of the hub’s other ports. Repeaters and hubs don’t examine the contents of the packets that they regenerate. If they did, they would be working at the Data Link layer, and not at the Physical layer. The network adapter (also called a network interface card or NIC) that’s
installed in each computer on the network is a Physical layer device. You can display information about the network adapter (or adapters) installed in a Windows computer by displaying the adapter’s Properties dialog box,




The Data Link Layer
The Data Link layer is the lowest layer at which meaning is assigned to the bits that are transmitted over the network. Data link protocols address things such as the size of each packet of data to be sent, a means of addressing each packet so that it’s delivered to the intended recipient, and a way to ensure that two or more nodes don’t try to transmit data on the network at the same time. The Data Link layer also provides basic error detection and correction to ensure that the data sent is the same as the data received. If an uncorrectable error occurs, the data link standard must specify how the node is to be informed of the error so that it can retransmit the data. At the Data Link layer, each device on the network has an address known as the Media Access Control address, or MAC address. This address is actually hard-wired into every network device by the manufacturer. MAC addresses are unique; no two network devices made by any manufacturer anywhere in the world can have the same MAC address. And once a device has been
manufactured, its MAC address can’t be changed. You can see the MAC address for a computer’s network adapter by opening a command window and running the ipconfig /all command One of the most import  functions of the Data Link layer is to provide a way for packets to be sent safely over the physical media without interference from other nodes attempting to send packets at the same time. The two most popular ways to do this are CSMA/CD and token passing. Ethernet networks use CSMA/CD, and Token Ring networks use token passing. Two types of Data Link layer devices are commonly used on networks:
bridges and switches. A bridge is an intelligent repeater that is aware of the MAC addresses of the nodes on either side of the bridge and can forward packets accordingly. A switch is an intelligent hub that examines the MAC address of arriving packets in order to determine which port to forward the packet to.



The Network Layer
The Network layer handles the task of routing network messages from one computer to another. The two most popular layer 3 protocols are IP (which is usually paired with TCP) and IPX (normally paired with SPX for use with Novell and Windows networks). Network layer protocols provide two important functions: logical addressing and routing. The following sections describe these functions.
 
Logical addressing
As you know, every network device has a physical address called a MAC address, which is assigned to the device at the factory. When you buy a network interface card to install into a computer, the MAC address of that card is fixed and can’t be changed. But what if you want to use some other addressing scheme to refer to the computers and other devices on your network? This is where the concept of logical addressing comes in; a logical address lets you access a network device by using an address that you assign. Logical addresses are created and used by Network layer protocols such as IP or IPX. The Network layer protocol translates logical addresses to MAC addresses. For example, if you use IP as the Network layer protocol, devices
on the network are assigned IP addresses such as 207.120.67.30. Because the IP protocol must use a Data Link layer protocol to actually send packets to devices, IP must know how to translate the IP address of a device to the device’s MAC address. Data Link layer addresses (or MAC addresses) are assigned at the factory and can’t be changed. Network layer addresses (or IP addresses) are assigned in the field and can be changed. You can use the ipconfig command to see the IP address of your computer. Although the exact format of logical addresses varies depending on the protocol being used, most protocols divide the logical address into two parts: a network address and a device address. The network address identifies which
network the device resides on, and the device address then identifies the device on that network. For example, in a typical IP address, such as 192.168.1.100, the network address is 192.168.1 and the device address (called a host address in IP) is 100. Similarly, IPX addresses consist of two parts: a network address and a node address. In an IPX address, the node address is the same as the MAC address.
As a result, IPX doesn’t have to translate between layer 3 and layer 2 addresses.


Routing
Routing comes into play when a computer on one network needs to send a packet to a computer on another network. In this case, a device called a router is used to forward the packet to the destination network. In some cases, a packet may actually have to travel through several intermediate networks in order to reach its final destination network. You can find out more about routers in Book I, Chapter 3. An important feature of routers is that you can use them to connect networks that use different layer 2 protocols. For example, a router can be used to send a packet from an Ethernet to a Token Ring network. As long as both networks support the same layer 3 protocol, it doesn’t matter if their layer 1 and layer 2 protocols are different.




The Transport Layer
The Transport layer is the layer where you’ll find two of the most well-known networking protocols: TCP (normally paired with IP) and SPX (normally paired with IPX). As its name implies, the Transport layer is concerned with the transportation of information from one computer to another. The main purpose of the Transport layer is to ensure that packets are transported reliably and without errors. The Transport layer does this task by establishing connections between network devices, acknowledging the receipt of packets, and resending packets that are not received or are corrupted when they arrive. In many cases, the Transport layer protocol divides large messages into smaller packets that can be sent over the network efficiently. The Transport layer protocol reassembles the message on the receiving end, making sure that all of the packets that comprise a single transmission are received so that no data is lost. For some applications, speed and efficiency are more important than reliability. In such cases, a connectionless protocol can be used. A  connectionless protocol doesn’t go to the trouble of establishing a connection before sending a packet. Instead, it simply sends the packet. TCP is a connectionoriented Transport layer protocol. The connectionless protocol that works alongside TCP is called UDP. Another important feature of the Transport layer protocols is name resolution. The Transport layer allows network nodes to be identified by names
rather than numbers. Infact, you can use the command NETSTAT /N to see the numeric network
addresses instead of the names.




The Session Layer
The Session layer establishes conversations known as sessions between networked devices. A session is an exchange of connection-oriented transmissions between two network devices. Each of these transmissions is handled by the Transport layer protocol. The session itself is managed by the Session layer protocol. A single session can include many exchanges of data between the two computers involved in the session. After a session between two computers has been established, it is maintained until the computers agree to terminate
the session. The session layer allows three types of transmission modes: 
✦ Simplex, in which data flows in only one direction.
✦ Half-duplex, in which data flows in both directions, but only in one direction at a time.
✦ Full-duplex, in which data flows in both directions at the same time.




The Presentation Layer
The Presentation layer is responsible for how data is represented to applications. Most computers — including Windows, UNIX, and Macintosh computers — use the American Standard Code for Information Interchange (ASCII) to represent data. However, some computers (such as IBM mainframe computers) use a different code, known as Extended Binary Coded Decimal Interchange Code (EBCDIC). ASCII and EBCDIC are not compatible with each other. To exchange information between a mainframe computer and a Windows computer, the Presentation layer must convert the data from ASCII to EBCDIC and vice versa.
Besides simply converting data from one code to another, the Presentation layer can also apply sophisticated compression techniques so that fewer bytes of data are required to represent the information when it’s sent over the network. At the other end of the transmission, the Presentation layer then uncompresses the data.
The Presentation layer can also scramble the data before it is transmitted and unscramble it at the other end by using a sophisticated encryption technique that even Sherlock Holmes would have trouble breaking.




The Application Layer
The highest layer of the OSI model, the Application layer, deals with the techniques that application programs use to communicate with the network. The name of this layer is a little confusing. Application programs such as Microsoft Office or QuickBooks aren’t a part of the Application layer. Rather, the Application layer represents the programming interfaces that application programs such as Microsoft Office or QuickBooks use to request network services.
Some of the better-known Application layer protocols are
✦ DNS (Domain Name System) for resolving Internet domain names.
✦ FTP (File Transfer Protocol) for file transfers.
✦ SMTP (Simple Mail Transfer Protocol) for e-mail.
✦ SMB (Server Message Block) for file sharing in Windows networks.
✦ NFS (Network File System) for file sharing in UNIX networks.
✦ Telnet for terminal emulation.







How data travels through the seven
layers.

The data begins
its journey when an end-user application sends data to another network computer. The data enters the network through an Application layer interface, such as SMB. The data then works its way down through the protocol stack. Along the way, the protocol at each layer manipulates the data by adding header information, converting the data into different formats, combining packets to form larger packets, and so on. When the data reaches the Physical layer protocol, it is actually placed on the network media (in other words, the cable) and sent to the receiving computer. When the receiving computer receives the data, the data works its way up through the protocol stack. Then, the protocol at each layer reverses the processing that was done by the corresponding layer on the sending computer. Headers are removed, data is converted back to its original format, packets that were split into smaller packets are recombined into larger messages,
and so on. When the packet reaches the Application layer protocol, it is delivered to an application that can process the data.

Understanding Protocols, Standards

  Understanding Protocols

A protocol is a set of rules that enable effective communications to occur. We encounter protocols every day. Computer networks depend upon many different types of protocols in order to work. These protocols are very rigidly defined, and for good reason. Network cards must know how to talk to other network cards in order to exchange information, operating systems must know how to talk to network cards in order to send and receive data on the network, and application programs must know how to talk to operating systems in order to know how to retrieve a file from a network server. Protocols come in many different types. At the lowest level, protocols define exactly what type of electrical signal represents a one and what type of signal represents a zero. At the highest level, protocols allow a computer user in the United States to send an e-mail message to another computer user in New Zealand. And in between are many other levels of protocols. You find out more about these levels of protocols (which are often called layers) in the section “The Seven Layers of the OSI Reference Model,”


Understanding Standards

A standard is an agreed-upon definition of a protocol. In the early days of computer networking, each computer manufacturer developed its own networking protocols. As a result, you weren’t able to easily mix equipment from different manufacturers on a single network. Then along came standards to save the day. Standards are industry-wide protocol definitions that are not tied to a particular manufacturer. With standard
protocols, you can mix and match equipment from different vendors. As long as the equipment implements the standard protocols, it should be able to coexist on the same network. Many organizations are involved in setting standards for networking. The five most important organizations are 

✦ American National Standards Institute (ANSI): The official standards organization in the United States. ANSI is pronounced An-See. 

✦ Institute of Electrical and Electronics Engineers (IEEE): An international organization that publishes several key networking standards; in particular, the official standard for the Ethernet networking system (known officially as IEEE 802.3). IEEE is pronounced Eye-triple-E.

✦ International Organization for Standardization (ISO): A federation of more than 100 standards organizations from throughout the world. If I had studied French in high school, I’d probably understand why the acronym for International Organization for Standardization is ISO, and not IOS. 

✦ Internet Engineering Task Force (IETF): The organization responsible for the protocols that drive the Internet. 

✦ World Wide Web Consortium (W3C): An international organization that handles the development of standards for the World Wide Web.

The Downside of Networking

 The Downside of Networking


 It’s not a personal computer anymore!
After you hook your computer up to a network, it’s not a personal computer anymore. You are now part of a network of computers, and in a way, you’ve given up one of the key things that made PCs so successful in the first place — independence.


Here are a few ways in which a network robs you of your independence:

✦ You can’t just indiscriminately delete files from the network. They may not be yours.
 
✦ The network forces you to be concerned about security. For example, a server computer has to know who you are before it will let you access its files. So you’ll have to know your user ID and password to access the network. This security feature is to prevent some 15-year-old kid from hacking his way into your office network via its Internet connection and stealing all your computer games.
 
✦ Just because you send something to a printer doesn’t mean it immediately starts to print. Someone else may have sent a big print job before you, so you’ll just have to wait.
 
✦ You may try to retrieve an Excel spreadsheet file from a network drive, only to discover that someone else is using it. You’ll just have to wait.
 
✦ If you find a really cool series of movies of astronauts walking on the moon at the NASA Web site and download them to the network server, you may get calls from angry coworkers complaining that no room is
left on the server’s drive for their important files. 

✦ Someone may pass a virus to you over the network. You may then accidentally infect other network users.
 
✦ You have to be careful about saving sensitive files on the server. If you write an angry note about your boss and save it on the server’s hard drive, your boss may find the memo and read it.
 
✦ If you want to access a file on a coworker’s computer but that person hasn’t yet arrived at work to turn on her computer, you have to go into her office and turn it on yourself. To add insult to injury, you have to
know that person’s password

✦ If your computer is a server, you can’t just turn it off when you’re finished using it. Someone else may be accessing a file on your hard drive or printing on your printer.




Network administration: Someone has to do it
Because so much can go wrong, even with a simple network, even small networks need to be managed. As a result, at least one person should be designated as the network manager (sometimes also called the network administrator). This way, someone is responsible for making sure that the network doesn’t fall apart or get out of control. For a small network, the network administrator doesn’t have to be a technical genius. In fact, some of the best network administrators are complete idiots when it comes to technical stuff. What’s important is that the manager be organized. The manager’s job is to make sure that plenty of space is available on the file server, that the file server is backed up regularly, that new employees can access the network, and so on.

Network Topology

Network Topology

The term network topology refers to the shape of how the computers and other network components are connected to each other. There are several different types of network topologies, each with advantages and disadvantages. In the following discussion of network topologies, I use two important
terms:
✦ Node: A node is a device that is connected to the network. For our purposes here, a node is the same as a computer. Network topology deals with how the nodes of a network are connected to each other.
✦ Packet: A packet is a message that is sent over the network from one node to another node. The packet includes the address of the node that sent the packet, the address of the node the packet is being sent to, and data.




Bus topology
The first type of network topology is called a bus, in which nodes are strung together in a line, as shown in Figure 1-1. Bus topology is commonly used for LANs.

The key to understanding how a bus topology works is to think of the entire network as a single cable, with each node “tapping” into the cable so that it can listen in on the packets being sent over that cable. If you’re old enough to remember party lines, you get the idea. In a bus topology, every node on the network can see every packet that’s sent on the cable. Each node looks at each packet to determine whether the packet is intended for it. If so, the node claims the packet. If not, the node ignores the packet. This way, each computer can respond to data sent to it and ignore data sent to other computers on the network. If the cable in a bus network breaks, the network is effectively divided into two networks. Nodes on either side of the break can continue to communicate with each other, but data can’t span the gap between the networks, so
nodes on opposite sides of the break can’t communicate with each other.




Star topology
In a star topology, each network node is connected to a central device called a hub or a switch, as shown in Figure 1-2. Star topologies are also commonly used with LANs. If a cable in a star network breaks, only the node connected to that cable is isolated from the network. The other nodes can continue to operate without
interruption — unless, of course, the node that’s isolated because of the break happens to be the file server.
You should be aware of the somewhat technical distinction between a hub and a switch. Simply put, a hub doesn’t know anything about the computers that are connected to each of its ports. So when a computer connected to the hub sends a packet to a computer that’s connected to another port, the hub sends a duplicate copy of the packet to all its ports. In contrast, a switch knows which computer is connected to  each of its ports. As a result, when a switch receives a packet intended for a particular computer, it sends
the packet only to the port that the recipient is connected to. Strictly speaking, only networks that use switches have a true star topology. If the network uses a hub, the network topology has the physical appearance of a star, but is actually a bus. That’s because when a hub is used, each computer on the network sees all the packets sent over the network, just like in a bus topology. In a true star topology, as when a switch is used, each computer sees only those packets that were sent specifically to it, as well
as broadcast packets that were specifically sent to all computers on the network.


Ring topology
A third type of network topology is called a ring, shown in Figure 1-3. In a ring topology, packets are sent around the circle from computer to computer. Each computer looks at each packet to decide whether the packet was intended for it. If not, the packet is passed on to the next computer in the ring.






















Mesh topology
A fourth type of network topology, known as mesh, has multiple connections between each of the nodes on the network, as shown in Figure 1-4. The advantage of a mesh topology is that if one cable breaks, the network can use an alternative route to deliver its packets. Mesh networks are not very practical in a LAN setting. For example, to network eight computers in a mesh topology, each computer would have to have seven network interface cards, and 28 cables would be required to connect each computer to the seven other computers in the network. Obviously, this scheme isn’t very scalable. However, mesh networks are common for metropolitan or wide area networks. These networks use devices called routers to route packets from network to network. For reliability and performance reasons, routers are usually arranged in a way that provides multiple paths between any two nodes on the network in a mesh-like arrangement.