A multicast address is a logical identifier for a group of hosts in a computer network that are available to process datagrams or frames intended to be multicast for a designated network service. Multicast addressing can be used in the link layer (layer 2 in the OSI model), such as Ethernet multicast, and at the internet layer (layer 3 for OSI) for Internet Protocol Version 4 (IPv4) or Version 6 (IPv6) multicast.

IPv4

IPv4 multicast addresses are defined by the most-significant bit pattern of 1110. This originates from the classful network design of the early Internet when this group of addresses was designated as Class D. The CIDR notation for this group is. The group includes the addresses from to. The address range is divided into blocks each assigned a specific purpose or behavior.

Notable IPv4 multicast addresses

The following table is a list of notable well-known IPv4 addresses that are reserved for IP multicasting and that are registered with the Internet Assigned Numbers Authority (IANA).

IPv6

Multicast addresses in IPv6 use the prefix. Based on the value of the flag bits, IPv6 multicast addresses can be Unicast-Prefix-based Multicast Addresses, Source-Specific Multicast Addresses, or Embedded RP IPv6 Multicast Addresses. Each of these types of multicast addresses have their own format and follow specific rules. Similar to a unicast address, the prefix of an IPv6 multicast address specifies its scope, however, the set of possible scopes for a multicast address is different. The 4-bit scope field (bits 12 to 15) is used to indicate where the address is valid and unique. The service is identified in the group ID field. For example, if refers to all Network Time Protocol (NTP) servers on the local network segment, then refers to all NTP servers in an organization's networks. The group ID field may be further divided for special multicast address types.

Notable IPv6 multicast addresses

The following table is a list notable IPv6 multicast addresses that are registered with IANA. To be included in some of the below multicast groups a client must send a Multicast Listener Discovery (MLD), a component of ICMPv6 suite, to join that group. For example, to listen to, a client must send a MLD report to the router, containing the multicast address, to indicate that it wants to listen to that group.

Ethernet

Ethernet frames with a value of 1 in the least-significant bit of the first octet of the destination MAC address are treated as multicast frames and are flooded to all points on the network. While frames with ones in all bits of the destination address are sometimes referred to as broadcasts, Ethernet generally does not distinguish between multicast and broadcast frames. Modern Ethernet controllers filter received packets to reduce CPU load, by looking up the hash of a multicast destination address in a table, initialized by software, which controls whether a multicast packet is dropped or fully received. The IEEE has allocated the address block to for group addresses for use by standard protocols. Of these, the MAC group addresses in the range of to are not forwarded by 802.1D-conformant MAC bridges.

802.11

802.11 wireless networks use the same MAC addresses for multicast as Ethernet.