CertiGuide to Network+  9  Chapter 0000:  Network Basics       9  III  Network Types by Geographic Scope

III  Network Types by Geographic Scope 1 2 Metropolitan Area Network (MAN)

LANs and Subnets

LAN’s larger than a handful of hosts are often broken down further into groupings referred to as subnets. The use of subnets allows an administrator to differentiate traffic between nearby, related machines, and traffic to those machines from other locations.

Why would you want to do this? Subnets help the administrator manage network bandwidth (the speed at which information can be transmitted across a network in a certain period of time). Each machine on a network adds packets (chunks of information organized in a standard format which flow from a source device to one or more destination devices) to the network when it communicates with any other device on the network. These use up a portion of the bandwidth available to the entire network.

Suppose two machines on the network are constantly swapping files. This amounts to many packets being added to the network by each one, as files are moved back and forth across the network. Networks have capacity limitations – once too many packets are on the network, the network slows down, much like freeways slow down when too many cars try to use them at the same time. Thus, the communication between these machines has the potential to slow down other machines on the network, even if those machines are only communicating with each other, and not communicating with the two file-swapping machines. If there are 50 other machines on the network, which rarely need to communicate with those two machines that constantly talk to each other, why slow them down just to accommodate the other two?

If you break this network into two separate subnets, with the two chatty machines on one subnet and the other machines on the second subnet, the packets for communication among the other 50 machines will have the network capacity on their subnet all to themselves, rather than having to share it with the other two machines. In effect, you use subnetting to give these two machines their own special-purpose lane on the expressway, with a big concrete barrier between it and the other lanes of the digital freeway. Only when either of the two chatty machines needs to communicate with one of the other 50, will its network traffic cross onto the other subnet.

Sometimes subnets are referred to as segments, a term which comes from the physical cabling world to denote a segment of cable. Typically segments are connected via a high-capacity backbone line, which is able to carry a greater amount of traffic at higher speeds, than a segment can.

III  Network Types by Geographic Scope 1 2 Metropolitan Area Network (MAN)