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Subnetting Basics


network address and create six networks from it? You would have to do something called sub- netting, because that’s what allows you to take one larger network and break it into a bunch of smaller networks.

There are loads of reasons in favor of subnetting. Some of the benefits include:

Reduced network traffic We all appreciate less traffic of any kind. Networks are no different. Without trusty routers, packet traffic could grind the entire network down to a near standstill. With routers, most traffic will stay on the local network; only packets destined for other net- works will pass through the router. Routers create broadcast domains. The more broadcast domains you create, the smaller the broadcast domains and the less network traffic on each net- work segment.

Optimized network performance This is a result of reduced network traffic.

Simplified management It’s easier to identify and isolate network problems in a group of smaller connected networks than within one gigantic network.

Facilitated spanning of large geographical distances Because WAN links are considerably slower and more expensive than LAN links, a single large network that spans long distances can create problems in every area listed above. Connecting multiple smaller networks makes the sys- tem more efficient.

In the following sections, I am going to move to subnetting a network address. This is the good part—ready?

IP Subnet-Zero

As I mentioned in the introduction, IP subnet-zero is not a new command, but in the pre-2005 CCNA objectives, Cisco didn’t cover it. This command allows you to use the first and last sub- net in your network design. For example, the Class C mask of 192 provides subnets 64 and 128 (discussed thoroughly later in this chapter), but with the ip subnet-zero command, you now get to use subnets 64, 128, plus 0 and 192.

Even though we don’t discuss the command line interface (CLI) until the next chapter, “Introduction to the Cisco IOS,” it’s important for you to be familiar with this command:

P1R1#sh running-config Building configuration... Current configuration : 827 bytes ! hostname Pod1R1 ! ip subnet-zero !

The router output above shows that the command ip subnet-zero is enabled on the router. Cisco has turned this command on by default starting with Cisco IOS version 12.x.

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