In classful IPv4 networking, Class D is reserved for multicast addresses.
The original classful scheme divided the 32-bit IPv4 address space into five classes based on leading bits. Classes A, B, and C were intended for unicast networks of different sizes. Class D used the leading bit pattern 1110 and covered 224.0.0.0 through 239.255.255.255, a range intended for one-to-many delivery rather than assigning addresses to individual hosts.
Multicast allows a sender to transmit one stream to a group of interested receivers. Routers and hosts can use multicast protocols to manage group membership and distribution, making it useful for applications such as routing protocols, streaming, conferencing, and service discovery. Class D addresses do not have the ordinary network-and-host division used by Classes A, B, and C.
Class E is the nearby source of another common confusion. It begins with 1111 and was reserved for experimental or future use, not multicast. Classful addressing itself became obsolete for Internet routing when Classless Inter-Domain Routing, or CIDR, was introduced in 1993. Even so, the Class D label remains a familiar shorthand for the IPv4 multicast range, while modern documentation more commonly describes it as 224.0.0.0/4.