IP Zip Codes: Network Numbers

IP Zip Codes: Network Numbers

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The conventions of IP addressing and IP address grouping make routing easy. In this example, all IP addresses on the Ethernet on the left begin with a first octet of 1. Similarly, all IP addresses that begin with 2 are on the Ethernet in the middle, and all IP addresses that begin with 3 should be on the Ethernet on the right. If you need to put a new computer on the leftmost Ethernet, you just need to give it an IP address that starts with 1, in addition to making sure that no other devices on that LAN are already using that IP address.

These conventions make routing easy. For instance, imagine the logic needed by R1 in Figure 10-6 to forward packets:

The routers do not have to know about every IP address in the network; they just need to know about each group of addresses. It's just like the idea that a postal sorting machine in Texas doesn't need to know every street address in Mason, Ohio; it just needs to know that if the zip code is 45040, the letter should be sent to Mason. When Hannah sends a packet to the http://www.fredsco.com web server (IP address 3.3.3.3), Hannah sends the packet to R1. R1 knows that all addresses that start with 3 can be reached by sending the packet to router R2, so R1 just forwards the packet to R2. R2, in turn, knows that all addresses that begin with 3 are on the Ethernet to its right, so it can send the packet directly to the web server on that Ethernet.

IP calls the group of IP addresses that share a common beginning to part of their addresses an IP network. Figure 10-6 shows three IP networks.

note The term "network" is used in a variety of ways and for slightly different meanings. Back in Chapter 1, "What Is a Network?," I defined a network as "a combination of hardware, software, and cabling, which together allow multiple computing devices to communicate with each other." When talking about IP addressing, the terms "network" and "IP network" have a different and more specific meaning, as will be explained in the upcoming pages of this chapter.

To talk about IP networks, you would use phrases such as "network 1" or "IP network 1" when referring to the group of IP addresses that start with a 1. However, when you're writing something down or typing something when you are working with a networking device, you use a special number that represents each network. That special number for each network is called an IP network number, or simply network number. Each network number represents a network.

Each IP network number has the same first part of the IP address as all the addresses in the network, but it has all 0s for the remaining octets. So, the network number looks like an IP address, but a host cannot use a network number as an IP address in the network. (Remember: A computer that has an IP address is referred to as a "host.") For instance, in the left-side Ethernet in Figure 10-6, all hosts begin their addresses with a single octet of 1. In that case, the network number is 1.0.0.0. Similarly, the other two network numbers in the figure are 2.0.0.0 and 3.0.0.0.