You may have noticed that the number of hosts listed in Table 2-1 is always two less than the number calculated. The reason for this discrepancy is that two special addresses can’t be assigned to a host. A host address of all ones is the broadcast address for a particular network, and a host address of all zeros is used by a host to temporarily identify itself (“this host”) until it has been assigned an IP address. Only 126 Class A networks exist because network 0 cannot be used, and network 127 is reserved for the loopback address that is used for testing interprocess communication. When a host sends a packet to 127.0.0.1, the data is not sent on the network but is returned immediately to the sending host.
The IP address blocks listed below have been reserved for private Internets.
11110.0.0.0 — 10.255.255.2550
1172.16.0.0 — 172.31.255.2550
192.168.0.0 — 192.168.255.255
These private IP addresses should never be advertised on the Internet because they can be used by any private Internet. If these addresses are used, then a technique such as network address translation would need to be used in the private Internet to be connected to the public Internet.
Classful IP address assignments can be extremely inefficient as the following design problem demonstrates. Assume we are designing a network for a campus that has approximately 1500 nodes or end-stations. Also assume that the predicted future growth of the network over the next five years will be no more than 5000 nodes. At first glance, it would seem that a Class B network would suffice for the current network requirements and also leave plenty of room for future growth. Having 1500-plus nodes (5000-plus in the future) would be a very large ethernet collision domain. If we want to limit the number of nodes on an ethernet segment to no more than 100, then we need 50 networks to accomplish our design. Regardless of which class of IP network addresses we decide to use (assuming we could choose any addresses we want), there will be an enormous waste of IP addresses as shown in Table 2-3.