IP Multicast Address Conventions

The IP address Class D is the reserved class for the multicast addresses by the Internet Assigned Number Authority (IANA). It has the most-significant bit pattern of 1110, 224.0.0.0/4 in CIDR notation. The multicast address scope ranges from 224.0.0.0 to 239.255.255.255.

In the multicast address range, multiple blocks of addresses are reserved for specific purposes which include the following:

Important Multicast Address Blocks

The following multicast address blocks are the most important to note:

1. Local Network Control Block (224.0.0.0/24) – used for protocol control traffic that is not forwarded out of a broadcast domain. For example:

• All hosts in this subnet – 224.0.0.1
• All routers in this subnet – 224.0.0.2
• All PIM routers – 224.0.0.13

2. Internetwork Control Block (224.0.1.0/24) – used for protocol control traffic that may be forwarded through the Internet. For example:

• Network Time Protocol (NTP) – 224.0.1.1
• Cisco-RP-Announce – 224.0.1.39
• Cisco-RP-Discovery – 224.0.1.40

The following lists some of the well-known multicast addresses from the local network and internetwork control blocks:

3. Source Specific Multicast (SSM) Block (232.0.0.0/8) – RFC 4607 describes SSM as a PIM extension, with SSM 232.0.0.0/8 as its default range. SSM only sends traffic to receivers from multicast sources that have expressed interest. It is generally used for one-to-many applications.

4. GLOP Block (233.0.0.0/8) – are globally scoped and statically assigned multicast addresses. It is created for domains having a 16-bit Autonomous System Number (ASN), in which the ASN is mapped to the domain. The ASN is expressed in the GLOP block’s middle two octets, X.Y, resulting in an assignment of 233.X.Y.0/24. RFC 3180 specifies the mapping and assignment. Domains with a 32-bit ASN can apply for Ad Hoc Block III block or use IPv6 multicast addresses instead.

5. Administratively Scoped Block (239.0.0.0/8) – RFC 2365 specifies that this address block is confined to a single local group or organization. These addresses are identical to the RFC 1918 reserved IP unicast ranges and will not be assigned by the IANA to other protocols or groups. Network administrators can use this range within their domain without conflicting with others anywhere on the Internet. SSM is assigned to the 232.0.0.0/8 range by default, but it is typically deployed in private networks using the 239.0.0.0/8 range.

Layer 2 Multicast Addresses

The MAC address is associated with a NIC on a LAN segment. Every multicast group address is mapped to a special MAC address, which allows Ethernet interfaces to assign multicast packets to a specific group. A LAN segment can comprise several streams. Based on the MAC address assigned to the multicast traffic, the receiver knows which traffic to transmit to the CPU for processing.

The multicast MAC address’s first 24 bits are always 01:00:5E, and the 25th bit is always 0. The first byte’s low-order bit is called the Individual/Group (I/G) bit or Unicast/Multicast bit. If the I/G bit is 1, it signifies that the frame is a multicast frame. The remaining 23 bits of the multicast MAC address are copied directly from the lower 23 bits of the multicast group IP address.

For example, the multicast IP address 224.0.1.1 is mapped to the 01:00:5E:00:01:01 multicast MAC address:

• The multicast IP address 224.0.1.1 in binary is 11100000.00000000.00000001.00000001.
• The MAC address in binary is 00000001.00000000.01011110 .00000000.00000001.00000001
• The multicast MAC address I/G bit is 1 (00000001).
• The 25th bit of the MAC address is 0.

The lower 23 bits from the IP address are copied to the multicast MAC address. The multicast IP address is defined by the most-significant bit pattern of 1110. Therefore, the first 4 bits are also fixed, leaving 5 bits that are not transferred to the MAC address. This means that 32 (2^5) multicast group addresses can overlap since they correspond to a single multicast MAC address. For example, the multicast IP addresses 224.0.1.1 and 239.255.1.1.

When a receiver intends to receive a specified multicast feed, it sends an IGMP join message using that feed’s multicast IP group address. The receiver reprograms its NIC to acknowledge the multicast MAC group address that corresponds to the group address.

For example, if a PC joins 224.0.1.1, it would reprogram its NIC to receive 01:00:5E:7F:01:01. If the PC receives multicast traffic sent to other multicast IP and MAC addresses, it will ignore the traffic to prevent unnecessary CPU cycles and processing.

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