Multicast Addressing

describes the multicast address scopes used by multicast to operate at Layer 2 and Layer 3.

• The Internet Assigned Number Authority (IANA) assigned the IP Class D address space 224.0.0.0/4 for multicast addressing. It includes addresses ranging from 224.0.0.0 to 239.255.255.255.
• The first 4 bits of this whole range start with 1110.

Class D Addressing

• Local network control block (224.0.0/24) Addresses in the local network control block are used for protocol control traffic that is not forwarded out a broadcast domain.

• Internetwork control block (224.0.1.0/24) Addresses in the internetwork control block are used for protocol control traffic that may be forwarded through the Internet.

Well–Known Reserved Multicast Addresses

• Source Specific Multicast (SSM) block (232.0.0.0/8): This is the default range used by SSM. SSM is a PIM extension described in RFC 4607.
• GLOP block (233.0.0.0/8): Addresses in the GLOP block are globally scoped statically assigned addresses.
• Administratively scoped block (239.0.0.0/8): described in RFC 2365, are limited to a local group or organization. Similar to the reserved IP unicast ranges (such as 10.0.0.0/8) and will not be assigned by the IANA.

Layer 2 Multicast Addressing

Every multicast group address (IP address) is mapped to a special MAC address that allows Ethernet interfaces to identify multicast packets to a specific group. A LAN segment can have multiple streams, and a receiver knows which traffic to send to the CPU for processing based on the MAC assigned to the multicast traffic.

• The first 24 bits of a multicast MAC always start with 01:00:5E.
• The low-order bit of the first byte is the individual/group bit (I/G) bit, also known as the unicast/multicast bit.
• When it is set to 1, it indicates that the frame is a multicast frame, and the 25th bit is always 0.
• The lower 23 bits of the multicast MAC address are copied from the lower 23 bits of the multicast group IP address.

  Out of the 9 bits from the multicast IP that are not copied into the multicast MAC, the high-order bits 1110 are fixed; that leaves 5 bits that are variable that are not transferred into the MAC. Because of this, there are 32 (25) multicast IP addresses that are not universally unique and could correspond to a single MAC.

Layer 2 Address Mapping Overlap

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