4

I see in many protocols we use 224 multicast address as like eigrp uses 224.0.0.10 ospf uses 224.0.0.5 224.0.0.6

15

Because 224.0.0.0/24 is the range assigned by IANA for local multicast - Local Network Control Block.

Addresses in this range are non-routable, they can only exist on a link, and cannot be forwarded by a router. These protocols only require multicast to operate within a single link, often to provide dynamic neighbour discovery and flooding of protocol messages between directly connected neighbours.

Individual addresses from this range are assigned by IANA and documented in the following table:

https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-1

The purpose of the Local Network Control Block is defined in RFC 5771:

  1. 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].

4.1. Assignment Guidelines

Pursuant to section 4.4.2 of [RFC2780], assignments from the Local Network Control Block follow an Expert Review, IESG Approval, or
Standards Action process. See IANA [IANA] for the current set of
assignments.

8

Outside of Karl's explanation, it's a holdover from the original class-based IP addressing scheme. As you know, the classes were as follows (based on the first octet):

  • Class A (0xxxxxxx): [0-127].0.0.0/8
  • Class B (10xxxxxx): [128.0-191.255].0.0/16
  • Class C (110xxxxx): [192.0.0-223.255.255].0/24
  • Class D (1110xxxx): 224.0.0.0-239.255.255.255
  • Class E (1111xxxx): 240.0.0.0-255.255.255.254
  • (Broadcast): 255.255.255.255

This is what RFC 870 had to say about class D originally:

Class D (highest-order bits 1-1-1)

All addresses in this class are reserved for future use, possibly in support of multicast services. They should be allocated to R&D use for the present.

This definition was updated in RFC 1166:

The fourth type of address, class D, is used as a multicast address. The four highest-order bits are set to 1-1-1-0.

So this address range has been considered for its current use since at least 1983 and assigned in 1990: the particulars of how multicast would work were not settled until later. Interestingly enough, "Class D" was any address starting with 111 in RFC 870; it wasn't defined as 1110 until RFC 988.

Why do so many of the multicast addresses start with 224? The earliest multicast documents that I could find didn't actually give any addresses and indicated that the "Assigned Numbers" document would specify them later. The first document that I could find with a well-known address was the IGMPv1 support document RFC 1054 Host Extensions for IP Multicasting (p. 11):

Informal Protocol Description

Multicast routers send Host Membership Query messages (hereinafter called Queries) to discover which host groups have members on their attached local networks. Queries are addressed to the all-hosts group (address 224.0.0.1), and carry an IP time-to-live of 1.u

The previous IGMPv0 document (RFC988) did not specify any well-known addresses at all (only making reference to "class D" and, potentially, "class E" addresses) (p. 2):

  1. HOST GROUP ADDRESSES

Host groups are identified by class D IP addresses, i.e. those with "1110" as their high-order four bits. The remaining 28 bits are unstructured as far as hosts are concerned. The addresses of well-known, permanent groups are to be published in "Assigned Numbers". Class E IP addresses, i.e. those with "1111" as their high-order four bits, are reserved for future addressing modes.

The related RFC 990 Assigned Numbers only had this for multicast addresses (p. 21):

Other Reserved Internet Addresses

* Internet Address Name Network References - ---------------- ---- ------- ---------- 224.000.000.000-239.255.255.255 Multicast [44,JBP] 240.000.000.000-255.255.255.255 Reserved [JBP]

RFC 1060 Assigned Numbers finally codified the initial well-known addresses (p. 19) from IGMPv1:

INTERNET MULTICAST ADDRESSES

Host Extensions for IP Multicasting (RFC-1112) [43] specifies the extensions required of a host implementation of the Internet Protocol (IP) to support multicasting. Current addresses are listed below.

224.0.0.0 Reserved [43,JBP] 224.0.0.1 All Hosts on this Subnet [43,JBP] 224.0.0.2 All Gateways on this Subnet (proposed) [JBP] 224.0.0.3 Unassigned [JBP] 224.0.0.4 DVMRP Routers [140,JBP] 224.0.0.5 OSPFIGP OSPFIGP All Routers [83,JXM1] 224.0.0.6 OSPFIGP OSPFIGP Designated Routers [83,JXM1] 244.0.0.7-244.0.0.255 Unassigned [JBP] 224.0.1.0 VMTP Managers Group [17,DRC3] 224.0.1.1 NTP Network Time Protocol [80,DLM1] 224.0.1.2 SGI-Dogfight [AXC] 224.0.1.3 Rwhod [SXD] 224.0.1.4 VNP [DRC3] 244.0.1.5-244.0.1.255 Unassigned [JBP] 224.0.2.1 "rwho" Group (BSD) (unofficial) [JBP] 232.x.x.x VMTP transient groups [17,DRC3]

Note that when used on an Ethernet or IEEE 802 network, the 23 low-order bits of the IP Multicast address are placed in the low- order 23 bits of the Ethernet or IEEE 802 net multicast address 1.0.94.0.0.0. See the next section on "IANA ETHERNET ADDRESS BLOCK".

  • I think you missed the point of the question. Since multicast addresses range from 224.0.0.0 to 239.255.255.255, the question was why so many multicast addresses start with 224, as opposed to some other number, like 225, 230, etc., which are also in the multicast range. – Ron Maupin Jul 21 '18 at 19:38
  • @RonMaupin Fair enough. I'll do some more research into usages of this space and update the answer, but it looks like RFC1131 (OSPF1) is a possible first contender for now. – ErikF Jul 21 '18 at 19:47
  • @RonMaupin It looks like IGMPv1 codified the addresses first. I've updated the answer. – ErikF Jul 21 '18 at 22:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.