Small- Medium-and Large IP Networks

Small, Medium, and Large IP Networks

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This internetwork still uses three different IP network numbers, as does Figure 10-6. By using one IP network number for each physical network, this internetwork conforms to the rules that allow easy routing. Each network sits on a LAN, with routers separating the networks. However, two of the networksClass B network 130.4.0.0 and Class C network 199.1.1.0are much smaller than the Class A networks used in Figure 10-6.

Similarly, Class B network 130.4.0.0 has a 2-octet network part, leaving 2 octets for the host part. The valid IP addresses in this network begin with 130.4.0.1 and proceed to 130.4.0.2, 130.4.0.3, 130.4.0.4, and so onin this case, there are 65,534 addresses in Class B network 130.4.0.0. With Class C network 199.1.1.0, the first valid host address is 199.1.1.1, the second is 199.1.1.2, and so on, with a total of 254 valid host addresses in the network.

The beauty of using one IP network per physical networksuch as one IP network per Ethernet LANis that routing works well whether you have 10 computers installed or 10,000. In Figure 10-7, R1 and R2 still need only three facts to route packets: where to send packets destined for network 8.0.0.0, where to send packets to network 199.1.1.0, and where to send packets destined for network 130.4.0.0.

Simply put, Class A networks allow for a ton of IP addresses in a single IP network, Class B networks allow for a pretty large number, and Class C networks allow for a smaller number of host IP addresses. The people who made up IP addressing chose three sizes of networks because one size doesn't fit all companies and organizations. Larger companies can use Class A, medium-sized companies can use Class B, and small companies can use Class C.