IANA Guidelines for IPv4 Multicast Address Assignments
How IANA assigns IPv4 addresses for group messaging
Albanna, Z., Almeroth, K., Meyer, D., and M. Schipper, "IANA Guidelines for IPv4 Multicast Address Assignments", RFC 3171, DOI 10.17487/RFC3171, August 2001, <https://www.rfc-editor.org/info/rfc3171>.
@misc{rfc3171,
series = {Request for Comments},
number = 3171,
howpublished = {RFC 3171},
publisher = {RFC Editor},
doi = {10.17487/RFC3171},
url = {https://www.rfc-editor.org/info/rfc3171},
author = {Z. Albanna and K. Almeroth and D. Meyer and M. Schipper},
title = {{IANA Guidelines for IPv4 Multicast Address Assignments}},
pagetotal = 8,
year = 2001,
month = aug,
} RFC text
Contents
Network Working Group Z. Albanna
Request for Comments: 3171 Juniper Networks
BCP: 51 K. Almeroth
Category: Best Current Practice UCSB
D. Meyer
Sprint
M. Schipper
IANA
August 2001
IANA Guidelines for IPv4 Multicast Address Assignments
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This memo provides guidance for the Internet Assigned Numbers
Authority (IANA) in assigning IPv4 multicast addresses.
1. Introduction
The Internet Assigned Numbers Authority (IANA) (www.iana.org) is
charged with allocating parameter values for fields in protocols
which have been designed, created or are maintained by the Internet
Engineering Task Force (IETF). RFC 2780 [RFC2780] provides the IANA
guidance in the assignment of parameters for fields in newly
developed protocols. This memo expands on section 4.4.2 of RFC 2780
and attempts to codify existing IANA practice used in the assignment
IPv4 multicast addresses.
The terms "Specification Required", "Expert Review", "IESG Approval",
"IETF Consensus", and "Standards Action", are used in this memo to
refer to the processes described in [RFC2434]. The keywords MUST,
MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED, SHALL, SHALL NOT,
SHOULD, SHOULD NOT are to be interpreted as defined in RFC 2119
[RFC2119].
In general, due to the relatively small size of the IPv4 multicast
addresses space, further assignment of IPv4 multicast address space
is recommended only in limited circumstances. Specifically, the IANA
should only assign addresses in those cases where the dynamic
selection (SDP/SAP), GLOP, SSM or Administratively Scoped address
spaces cannot be used. The guidelines described below are reflected
in http://www.iana.org/numbers.html.
2. Definition of Current Assignment Practice
Unlike IPv4 unicast address assignment, where blocks of addresses are
delegated to regional registries, IPv4 multicast addresses are
assigned directly by the IANA. Current assignments appear as follows
[IANA]:
224.0.0.0 - 224.0.0.255 (224.0.0/24) Local Network Control Block
224.0.1.0 - 224.0.1.255 (224.0.1/24) Internetwork Control Block
224.0.2.0 - 224.0.255.0 AD-HOC Block
224.1.0.0 - 224.1.255.255 (224.1/16) ST Multicast Groups
224.2.0.0 - 224.2.255.255 (224.2/16) SDP/SAP Block
224.252.0.0 - 224.255.255.255 DIS Transient Block
225.0.0.0 - 231.255.255.255 RESERVED
232.0.0.0 - 232.255.255.255 (232/8) Source Specific Multicast
Block
233.0.0.0 - 233.255.255.255 (233/8) GLOP Block
234.0.0.0 - 238.255.255.255 RESERVED
239.0.0.0 - 239.255.255.255 (239/8) Administratively Scoped
Block
The IANA generally assigns addresses from the Local Network Control,
Internetwork Control, and AD-HOC blocks. Assignment guidelines for
each of these blocks, as well as for the Source Specific Multicast,
GLOP and Administratively Scoped Blocks, are described below.
3. 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 off link. Examples of this
type of use include OSPFIGP All Routers (224.0.0.5) [RFC2328].
3.1. Assignment Guidelines
Pursuant to section 4.4.2 of RFC 2780 [RFC2780], assignments from the
Local Network Control block follow an Expert Review, IESG Approval or
Standards Action process. See [IANA] for the current set of
assignments.
4. Internetwork Control Block (224.0.1/24)
Addresses in the Internetwork Control block are used for protocol
control that must be forwarded through the Internet. Examples
include 224.0.1.1 (NTP [RFC2030]) and 224.0.1.68 (mdhcpdiscover
[RFC2730]).
4.1. Assignment Guidelines
Pursuant to section 4.4.2 of RFC 2780 [RFC2780], assignments from the
Internetwork Control block follow an Expert Review, IESG Approval or
Standards Action process. See [IANA] for the current set of
assignments.
5. AD-HOC Block (224.0.2.0/24 - 224.0.255.0/24)
Addresses in the AD-HOC block have traditionally been assigned for
those applications that don't fit in either the Local or Internetwork
Control blocks. These addresses are globally routed and are
typically used by applications that require small blocks of
addressing (e.g., less than a /24).
5.1. Assignment Guidelines
In general, the IANA SHOULD NOT assign addressing in the AD-HOC
Block. However, the IANA may under special special circumstances,
assign addressing from this block. Pursuant to section 4.4.2 of RFC
2780 [RFC2780], assignments from the AD-HOC block follow an Expert
Review, IESG Approval or Standards Action process. See [IANA] for
the current set of assignments.
6. SDP/SAP Block (224.2/16)
Addresses in the SDP/SAP block are used by applications that receive
addresses through the Session Announcement Protocol [RFC2974] for use
via applications like the session directory tool (such as SDR [SDR]).
6.1. Assignment Guidelines
Since addresses in the SDP/SAP block are chosen randomly from the
range of addresses not already in use [RFC2974], no IANA assignment
policy is required. Note that while no additional IANA assignment is
required, addresses in the SDP/SAP block are explicitly for use by
SDP/SAP and MUST NOT be used for other purposes.
7. Source Specific Multicast Block (232/8)
The Source Specific Multicast (SSM) is an extension of IP Multicast
in which traffic is forwarded to receivers from only those multicast
sources for which the receivers have explicitly expressed interest,
and is primarily targeted at one-to-many (broadcast) applications.
Note that this block as initially assigned to the VMTP transient
groups [IANA].
7.1. Assignment Guidelines
Because the SSM model essentially makes the entire multicast address
space local to the host, no IANA assignment policy is required.
Note, however, that while no additional IANA assignment is required,
addresses in the SSM block are explicitly for use by SSM and MUST NOT
be used for other purposes.
8. GLOP Block (233/8)
Addresses in the GLOP block are globally scoped statically assigned
addresses. The assignment is made by mapping a domain's autonomous
system number into the middle two octets of 233.X.Y.0/24. The
mapping and assignment is defined in [RFC2770].
8.1. Assignment Guidelines
Because addresses in the GLOP block are algorithmically pre-assigned,
no IANA assignment policy is required. In addition, RFC 3138
[RFC3138] delegates assignment of the GLOP sub-block mapped by the
RFC 1930 [RFC1930] private AS space (233.252.0.0 - 233.255.255.255)
to the Internet Routing Registries. Note that while no additional
IANA assignment is required, addresses in the GLOP block are
assigned for use as defined in RFC 2770 and MUST NOT be used for
other purposes.
9. Administratively Scoped Address Block (239/8)
Addresses in the Administratively Scoped Address block are for local
use within a domain and are described in [RFC2365].
9.1. Assignment Guidelines
Since addresses in this block are local to a domain, no IANA
assignment policy is required.
9.1.1. Relative Offsets
The relative offsets [RFC2365] are used to ensure that a service can
be located independent of the extent of the enclosing scope (see RFC
2770 for details). Since there are only 256 such offsets, the IANA
should only assign a relative offset to a protocol that provides an
infrastructure supporting service. Examples of such services include
the Session Announcement Protocol [RFC2974]. Pursuant to section
4.4.2 of RFC 2780 [RFC2780], assignments of Relative Offsets follow
an Expert Review, IESG Approval or Standards Action process. See
[IANA] for the current set of assignments.
10. Annual Review
Given the dynamic nature of IPv4 multicast and its associated infra-
structure, and the previously undocumented IPv4 multicast address
assignment guidelines, the IANA should conduct an annual review of
currently assigned addresses.
10.1. Address Reclamation
During the review described above, addresses that were mis-assigned
should, where possible, be reclaimed or reassigned.
The IANA should also review assignments in the AD-HOC, DIS Transient
Groups, and ST Multicast Groups blocks and reclaim those addresses
that are not in use on the global Internet (i.e, those applications
which can use SSM, GLOP, or Administratively Scoped addressing, or
are not globally routed).
11. Use of IANA Reserved Addresses
Applications MUST NOT use addressing in the IANA reserved blocks.
12. Security Considerations
The assignment guidelines described in this document do not alter the
security properties of either the Any Source or Source Specific
multicast service models.
13. Acknowledgments
The authors would like to thank Joe St. Sauver, John Meylor, Randy
Bush, and Thomas Narten for their constructive feedback and comments.
14. Authors' Addresses
Zaid Albanna
1149 N. Mathilda Ave
Sunnyvale, CA. 94089
EMail: zaid@juniper.net
Kevin Almeroth
UC Santa Barbara
Santa Barbara, CA.
EMail: almeroth@cs.ucsb.edu
David Meyer
Sprint E|Solutions
EMail: dmm@sprint.net
Michelle Schipper
IANA Administrator
Internet Assigned Numbers Authority
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
EMail: iana@iana.org
15. References
[IANA] http://www.iana.org/numbers.html
[RFC1190] Topolcic, C., "Experimental Internet Stream Protocol,
Version 2 (ST-II)", RFC 1190, October 1990.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
RFC 1930, March 1996.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2030] Mills, D., "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 2030, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
RFC 2365, July 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2730] Hanna, S., Patel, B. and M. Shah, "Multicast Address
Dynamic Client Allocation Protocol (MADCAP), RFC 2730,
December 1999.
[RFC2770] Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8", RFC
2770, February 2000.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers", BCP
37, RFC 2780, March 2000.
[RFC2908] Thaler, D., Handley, M. and D.Estrin, "The Internet
Multicast Address Allocation Architecture", RFC 2908,
September 2000.
[RFC2909] Thaler, D., Handley, M. and D. Estrin, "The Multicast
Address-Set Claim (MASC) Protocol", RFC 2909, September
2000.
[RFC2974] Handley, M., Perkins, C. and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC3138] Meyer, D., "Extended Assignments in 233/8", RFC 3138, June
2001.
[SDR] http://www-mice.cs.ucl.ac.uk/multimedia/software/
16. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Introduction
Multicast is a way to send a single message to multiple recipients at once, like a group text message instead of individual ones. Each multicast group has an IPv4 address, and when someone sends to that address, all computers listening to that group receive the message.
Because the IPv4 multicast address space is much smaller than the unicast space (regular one-to-one addresses), assignments must be managed carefully. This RFC codifies IANA's practice for assigning multicast addresses, replacing informal procedures with explicit guidelines. It recommends that IANA only assign addresses in cases where dynamic methods (which don't require central assignment) cannot be used.
Multicast is like a radio station: one broadcast, many listeners. There are only so many frequency channels, so the address librarian needs rules about who gets a channel.
§2The Multicast Address Space
The IPv4 multicast address range (224.0.0.0 to 239.255.255.255) is divided into blocks, each serving a different purpose. The Local Network Control Block (224.0.0/24) contains addresses like 224.0.0.5 (OSPF routers), used only on the local network link and not forwarded by routers. The Internetwork Control Block (224.0.1/24) contains addresses like 224.0.1.1 (NTP) that are forwarded across the Internet.
The AD-HOC Block (224.0.2.0 through 224.0.255.0) is for applications that don't fit other categories. The Source Specific Multicast Block (232/8) is for SSM, where receivers only get traffic from explicitly chosen senders. The GLOP Block (233/8) uses a clever algorithm: a company's autonomous system number is encoded into the address, making addresses algorithmically available without central assignment. The Administratively Scoped Block (239/8) is for local use within organizations. Several other blocks are reserved for future use.
The multicast address space is divided into neighborhoods. Some are for talking to neighbors on the same link. Some are for talking across the Internet. Some let companies make their own addresses using a formula.
§3How Addresses Are Assigned
For the Local Network Control Block and Internetwork Control Block, new assignments require Expert Review, IESG Approval, or Standards Action. These blocks are reserved for protocol control traffic, and assigning one requires justifying why the application cannot use other methods.
For the AD-HOC Block, the RFC strongly discourages new assignments; IANA should only assign when dynamic methods fail. For SDP/SAP (Session Announcement Protocol), addresses are chosen randomly by users, so no IANA assignment policy is needed. For SSM and GLOP, addresses are algorithmically or dynamically assigned, eliminating the need for centralized allocation. The Administratively Scoped Block requires no IANA assignment since addresses are for local use.
Some addresses need permission from IANA. Some can be picked by users. Some are calculated automatically. The RFC says to use the automatic methods first.
§4Relative Offsets and Annual Reviews
Within the Administratively Scoped Block, there are special relative offsets: fixed addresses that represent services like session announcement. Since only 256 offsets exist, IANA should assign them only to services providing infrastructure support. New relative offset assignments require Expert Review, IESG Approval, or Standards Action.
The RFC recommends that IANA conduct annual reviews of multicast address assignments, reclaiming addresses that were mis-assigned or are no longer in use. Addresses in the AD-HOC, DIS Transient, and ST Multicast blocks should be reclaimed if they are unused on the global Internet or could instead use SSM, GLOP, or Administratively Scoped addressing.
Sometimes IANA hands out addresses that don't get used. Once a year, IANA should check and take back addresses from apps that aren't using them or should have picked a different way.
§5Avoiding Misuse and Interoperability
The RFC notes that applications MUST NOT use addressing in the IANA reserved blocks. The assignment guidelines do not alter the underlying security properties of multicast; they simply organize how addresses are distributed. The guidelines protect interoperability by ensuring different applications do not accidentally use the same address and by preventing applications from consuming the scarce static address pool unnecessarily.
By promoting dynamic allocation methods, the RFC reduces contention and simplifies the process for applications that need multicast addressing, while centralizing IANA review on only the most critical cases (control protocols that cannot use dynamic allocation).
The main risk is apps fighting over addresses or wasting rare addresses on things that could pick their own. These rules help prevent that.