IP classes are used to assist in assigning IP addresses to networks with different size requirements. The IPv4 IP address space can be subdivided into five address classes called Class A, B, C, D, and E. Each IP class consists of a contiguous subset of the overall IPv4 address range. One such class is reserved only for multicast addresses, which is a type of data transmission where more than one computer is addressed information at once.
IP Address Classes and Numbering
The values of the leftmost four bits of an IPv4 address determine its class. For example, all Class C addresses have the leftmost three bits set to 110, but each of the remaining 29 bits may be set to either 0 or 1 independently (as represented by an x in these bit positions): The table below describes the IP address values and ranges for each class. Note that some of the IP address space is excluded from Class E for special reasons as described further below. A special type of IP address is the limited broadcast address 255.255.255.255. A network broadcast involves delivering a message from one sender to many recipients. Senders direct an IP broadcast to 255.255.255.255 to indicate all other nodes on the local area network should pick up that message. This broadcast is “limited” in that it does not reach every node on the internet; only nodes on the LAN. Internet Protocol officially reserves the entire range of addresses from 255.0.0.0 through 255.255.255.255 for broadcast, and this range should not be considered part of the normal Class E range.
IP Address Class D and Multicast
The IPv4 networking standard defines Class D addresses as reserved for multicast. Multicast is a mechanism in Internet Protocol for defining groups of client devices and sending messages only to that group rather than to every device on the LAN (broadcast) or just one other node (unicast). Multicast is mainly used on research networks. As with Class E, Class D addresses should not be used by ordinary nodes on the internet.
