Small, Medium, and Large IP Networks
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 22.214.171.124
and Class C network 126.96.36.199are much smaller than the Class A networks used in
For a closer look, consider network 188.8.131.52 for a moment. It is
a Class A network, which means that only 1 byte is used for the network part of
the address. As mentioned before, all hosts in network 184.108.40.206 begin with 8. So,
220.127.116.11 is a valid IP address in that network, as is 18.104.22.168, 22.214.171.124, 126.96.36.199,
188.8.131.52, and so on. The actual number of valid host addresses in Class A network
184.108.40.206 is 16,777,214.
Similarly, Class B network 220.127.116.11 has a 2-octet network
part, leaving 2 octets for the host part. The valid IP addresses in this network
begin with 18.104.22.168 and proceed to 22.214.171.124, 126.96.36.199, 188.8.131.52, and so onin
this case, there are 65,534 addresses in Class B network 184.108.40.206. With Class C
network 220.127.116.11, the first valid host address is 18.104.22.168, the second is
22.214.171.124, 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 126.96.36.199, where to send packets to network
188.8.131.52, and where to send packets destined for network 184.108.40.206.
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.