IPv4 (Internet protocol version four) is the current mainstream addressing protocol for the Internet and other TCP/IP (transmission control protocol/Internet protocol) networks. Each 32-bit binary (i.e., base 2) IP address in this scheme is commonly represented in dotted decimal notation by four 8-bit fields, called octets, such as 18.104.22.168 and 127.0.0.1, in order to be easier to read and remember.
IPv4 contains three main categories of networks, defined by the first three bits in the IP address: Class A, Class B and Class C. The number of bits used to identify the network and the host (i.e., computer attached to the network) vary according to the network class of the address.
The network class can be easily identified by the first octet, which is within the range of 192 to 233 for Class C networks, as compared with 1 to 126 for Class A networks and 128 to 191 for Class B networks. 127 is reserved for the loopback address, which is used on single computers and not physically connected to any network.
Class C networks are the most numerous and the smallest of the three classes. After the high-order bits, the next 21 bits in Class C IP addresses are used to identify the network, and the final eight bits are used to identify the host. This makes possible a maximum of 2,097,152 networks, each of which can have a maximum of 254 hosts.
Created December 11, 2005.