An octet is a group of exactly eight bits. An IPv4 address contains 32 bits, divided into four octets and written as four decimal numbers separated by periods: dotted decimal notation. The term octet is preferred for IPv4 because a byte has not always meant exactly eight bits. section 7.3.1
Use these points when reading IPv4 addresses:
- Each octet contains eight binary positions, whose values are 1, 2, 4, 8, 16, 32, 64, and 128. section 7.4
- An 8-bit octet represents 256 possible values, from 0 (00000000) through 255 (11111111). section 7.4
- A prefix length such as /24 identifies the first 24 bits, or first three octets, as the network portion; the final octet is the host portion. section 7.3.1
IPv4 octets in a LAN
In 192.168.100.100/24, the four octets are 192, 168, 100, and 100. The first three octets identify the shared network portion, while hosts on that LAN use different values in the final octet, such as 1, 178, or 234. section 7.3.1
Figure 7.7 An IPv4 address written in both dotted decimal and binary. The 32-bit address is split into four octets consisting of 8 bits each. The address is divided into two parts: the network portion and the host portion. The prefix length indicates the size of the network portion in bits, and the remainder is the host portion.
Compare the dotted-decimal and binary forms of an IPv4 address, including its network and host portions. section 7.3.1
