TCP/IP Version 4 Address

A TCP/IP version 4 address consists of four numbers (0 through 255) separated by periods. For example, 128.1.15.95 is a valid TCP/IP address. The numbers 0 and 255 may be reserved for special purposes; therefore, avoid using them in TCP/IP addresses unless you are sure that the host portion of the TCP/IP address (in binary) is not all 0s or all 1s (These addresses are reserved by the TCP/IP architecture to designate the network, host bits all 0s, and for use as a TCP/IP broadcast, host bits all 1s).

The Internet is composed of thousands of smaller networks that are joined together by TCP/IP routers. Similarly, your own TCP/IP network may be composed of many smaller LANs. These LANs may be of different physical types, such as Ethernet or token ring and are referred to as network segments. Generally, each network segment has its own unique network address.

The TCP/IP address assigned to your machine is actually composed of the unique address for your network segment and the unique address for your machine within your segment. That is, the address is composed of a network portion (the real network address plus any subnet address) and a host portion:

TCP/IP address = network address + host address

The four numbers separated by periods in your TCP/IP address may be divided into a network portion and a host portion in many different ways. There are five classes of TCP/IP networks; we will discuss three of them here. The network class is determined by the number in the first position of the address:

o Class A : 1-127

o Class B : 128-191

o Class C : 192-223

Class A addresses normally use only the first part of the TCP/IP address as its network address. For example, IBM's main network address is 9. This falls into the Class A range of numbers.

Class B addresses normally use the first two parts of the TCP/IP address for the network portion of the address. Network 128.1 is a valid Class B network.

Class C addresses use the first three parts of the address, for example, 192.10.25. In this case, only the last part of the TCP/IP address is available for host addresses. Therefore, your Class C network can have only hosts with numbers of 1 through 254. This limits the size of your network to 254 hosts.