A TCP/IP network routes a packet according to the destination IP address, an address provided by the IP protocol on the sending host. IP addresses are 32-bit numbers that uniquely identify every host computer connected to the Internet. This 32-bit address is typically represented as four numbers that are separated by dots. For example, the following is an IP address:
137.65.208.51Because each of the four numbers is represented by an eight-bit field (or octet), each number must fall between 0 and 255. The last field, however, can only be assigned numbers from 1-254 for a class C address, or for a class B address with an 8-bit subnet. This is because the host part of the address cannot be all 0's or all 1's. Classes are discussed in ``Network address''.
The tricky part about IP addresses is that the network address (the part of the address that identifies your network) and the host address (the part that identifies an individual host on your network) must all fit into this 32-bit address space. The number of hosts you can configure for your network varies because the size of the network address is different for different network ``classes.'' To complicate matters further, if your network consists of several physical networks you must further partition your host address to use part of it as a subnet address.
Descriptions of the parts of an IP address (network address, host address, and optional subnet address) follow.