IP Address Classes
To accommodate different sizes of networks and to aid in classifying them, IP addresses are divided into categories that are called classes. Assigning IP addresses to classes is known as classful addressing. The classes were determined by the IANA.
As mentioned before, each IP address consists of a network ID and a host ID. Each address class uses a fixed number of octets (that is, a fixed set of bits) to indicate the network ID. The remaining octets of the 32-bit IP address are used for host addresses. A bit or bit sequence at the start of a classful IP address determines the class of the address. This starting bit or bit sequence is known as the MSB.
The table below shows three of the five IP address classes.
- Class A: The first 0 of a Class A address is always in the first bit (MSB) position. Because the first bit is a 0, the lowest number that can be represented is 00000000 (decimal 0), and the highest number that can be represented is 01111111 (decimal 127). However, these 2 network numbers, 0 and 127, are reserved and cannot be used as network addresses. An address that has a value in the range of 1 to 126 in the first octet of the 32-bit number is a Class A address.
- Class B: The first 0 of a Class B address is always in the second bit position. Starting the first octet with binary 10 ensures that the Class B space is separated from the upper levels of the Class A space. The remaining 6 bits in the first octet can be populated with either 1s or 0s. Therefore, the lowest number that can be represented with a Class B address is 10000000 (decimal 128), and the highest number that can be represented is 10111111 (decimal 191). An address that has a value in the range of 128 to 191 in the first octet is a Class B address.
- Class C: The first 0 of a Class C address is always in the third bit position. Therefore, the lowest number that can be represented is 11000000 (decimal 192), and the highest number that can be represented is 11011111 (decimal 223). If an address contains a number in the range of 192 to 223 in the first octet, it is a Class C address.