Unicast-Prefix-Based IPv4 Multicast Addresses
Extracted elements (12)
GLOP (RFC 3180) embeds a 2-byte AS number in 233/8, yielding only 256 addresses per AS and failing with 4-byte AS numbers (RFC 4893). The UBM mechanism instead ties multicast space to unicast prefixes at subnet granularity, eliminating the need for an inter-domain multicast address allocation protocol and aligning IPv4 practice more closely with IPv6 RFC 3306.
Providing multicast space at subnet rather than AS granularity means address assignments within a range need only be coordinated within the subnet, reducing administrative scope and potential conflicts.
This document does not obsolete or update RFC 3180 (GLOP). GLOP remains required for organizations with prefix allocations more specific than /24, and organizations may use both GLOP and UBM allocations simultaneously.
Because global unicast addresses are not permanently assigned, UBM addresses MUST NOT be hard-coded in applications.
The embedded unicast prefix in a UBM address MUST be a global unicast prefix; loopback, multicast, link-local, or private-use IP address space are not permitted.
A multicast address with the prefix 234/8 indicates a Unicast-Based Multicast (UBM) address. The remaining 24 bits embed the organization's unicast prefix (bits 8 through N) followed by a Group ID (bits N+1 through 31).
Organizations with a prefix length longer than /24 do not receive any multicast address space from this mechanism and must use another method (e.g., GLOP or MADCAP).
Organizations with a unicast prefix of /24 or shorter may derive a multicast prefix by appending their unicast prefix to the common 234/8. Any remaining bits may be assigned by whatever mechanism the organization chooses.
The organization assigned a UBM address can be identified by taking the multicast address, shifting it left by 8 bits, and looking up who holds the resulting unicast address space.
IANA has assigned the 234/8 block from the global IPv4 multicast address space for unicast-prefix-based multicast (UBM) addresses as defined by this specification.
The same well-known intra-domain security techniques applicable to GLOP apply to UBM. Dynamic allocation within a prefix may reduce exposure to denial-of-service attacks because nodes compete for addresses only within an individual organization's prefix rather than across an entire AS.
UBM address layout: bits 0–7 are the fixed value 234; bits 8 through N carry the unicast prefix of the organization; bits N+1 through 31 carry the Group ID assigned by the organization. Total address length is 32 bits (standard IPv4).