IPv4 address classes categorize IP addresses into five classes: A, B, C, D, and E. Classes A, B, and C are usable for unicast (host-to-host) communication. Class A supports a large number of hosts in a network, while class B and C are suitable for medium and small-sized networks, respectively.
Classes D and E serve different purposes. Class D addresses are reserved for multicast communication, allowing data to be sent to multiple recipients simultaneously. Class E addresses are reserved for experimental or research purposes and are not used in regular network communication.
As you can see in the image above, IPv4 address classes A, B, and C differ in the number of octets allocated for network and host IDs:
- Class A: Class A uses 1 octet as network ID and 3 octets for host ID, with the first bit fixed at 0 in the network ID, meaning that the supported number range is between 1 and 127. This class provides a total of 16,777,216 million IPs.
- Class B: Class B uses 2 octets for network ID and 2 octets for host ID, with the first 2 bits fixed at 10 in the network ID, meaning that the supported number range is between 128 and 191 in the first octet. It provides a total of 65,536 IPs.
- Class C: Class C uses 3 octets for network ID and 1 octet for host ID, with the first 3 bits fixed at 110 in the network ID, meaning that the supported number range is between 192 and 223 in the first octet. This class provides a total of 256 IPs.
These distinctions allow efficient allocation of IP addresses for different-sized networks, with Class A suitable for large networks, Class B for medium-sized ones, and Class C for smaller networks.