Under TCP/IP, every system on a network is assigned a 32-bit IP address. Instead of representing these bytes in hexadecimal, however, they are normally written in the format w.x.y.z, where each variable is replaced with a decimal number between 0 (hex 00) and 255 (hex FF). For example, 18.104.22.168 is a valid IP address. The IP address is further broken down to identify a logical network address and a system. For example, 22.214.171.124 identifies a specific system, "12," within a specific network, "192.0.0." Similarly, address 126.96.36.199 identifies system "20.12" within network "128.10."
You can create an IP address in one of three formats, called classes. Each class breaks the address down into network and host system components differently. The three classes are:
- Class A: Uses the format "network.host.host.host," with the network component falling between 0 and 127 (exclusive of 0 and 127), and the host component being greater than 0. For example, in the address 188.8.131.52, "64" identifies the network, and "0.1.12" the host system.
- Class B: Uses the format "network.network.host.host," with the first network component falling in the range from 128 through 191 (including 128 and 191), and the host component being greater than 0. For example, in the address 184.108.40.206, "130.101" identifies the network, and "0.68" the host system.
- Class C: Uses the format "network.network.network.host," with the first network component falling in the range from 192 through 223 (including 192 and 223), and the host component being greater than 0. For example, in the address 220.127.116.11, "200.1.1" identifies the network, and "37" the host system.
- Class D: Uses addresses beginning with a number in the range from 224 through 239 (inclusive).
All three formats identify a particular host system within a specific network. A fourth type of address--a Class D address--is used for multicast addresses. Multicast addresses are intended for multiple systems that may reside on different networks; therefore, Class D addresses have no network or host components.