Multicast Addressing Guide for IP & MAC Multicast Addresses

#readwise # Multicast Addressing: Guide for IP & MAC Multicast Addresses ![rw-book-cover](https://learnwithsalman.com/wp-content/uploads/2025/01/Multicast-Addressing.png) ## Metadata - Author: [[Salman Alhiary]] - Full Title: Multicast Addressing: Guide for IP & MAC Multicast Addresses - URL: https://learnwithsalman.com/multicast-addressing/ ## Summary Multicast addressing allows a single source to send data to multiple receivers in a network. It uses special group addresses for traffic, which are never assigned to devices. The article explains both IPv4 and MAC multicast addressing, detailing how they work together. Understanding these concepts can simplify network management for engineers and administrators. ## Highlights IPv4 Multicast addresses are identified by the value of the first octet. Therefore, the first 4 bits are always (`1110xxxx`). IPv4 Multicast uses Class “D” addresses (`224.0.0.0/4`); the range is (`224.0.0.0 – 239.255.255.255`). ([View Highlight](https://read.readwise.io/read/01jnne5pjsjrx8d74t80v40hq2)) ^9zn1xx --- The Internet Assigned Numbers Authority ([IANA](https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml)) has assigned several ranges of Multicast IP addresses for specific types: | Type | Block | Description | | -------------------------------------------------- | ------------------------------------------------------------------------------------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Permanent Groups – Local Network Control Block | `224.0.0.0 - 224.0.0.255` | Addresses from this range are assigned for network protocols on a local segment. Routers do not forward packets with destination addresses used from this range. | | Permanent Groups – Internetwork Control Block | `224.0.1.0 - 224.0.1.255` | IANA assigns addresses from this range for the network protocols that are forwarded to the entire network. Routers forward packets with destination addresses used from this range. | | Source Specific Multicast (SSM) | `232.0.0.0 - 232.255.255.255` | IANA has allocated this range for SSM applications and protocols. The purpose of these applications is to allow a host to select a source for the Multicast group. | | Organization-Local Scope (Administratively Scoped) | `239.0.0.0 - 239.255.255.255` | IANA has assigned the range to be used in private Multicast domains, much like the IP unicast ranges defined in RFC-1918. Network administrators are free to use Multicast addresses in this range; however, they must configure their Multicast routers to ensure that Multicast traffic in this address range does not leave their Multicast domain boundaries. | | GLOP addressing | `233.0.0.0 - 233.255.255.255` | IANA has reserved this range on an experimental basis. It can be used by any organization that owns a registered autonomous system number (ASN) to create 256 global Multicast addresses that can be owned and used by the entity. By using a value of `233` for the first octet and by using the ASN for the second and third octets, a single autonomous system can create globally unique Multicast addresses as defined in the GLOP addressing ([RFC-3180](https://datatracker.ietf.org/doc/html/rfc3180)). | | AD-HOC Blocks, reserved, or transient groups | `224.0.2.0 - 224.0.255.255` `224.3.0.0 - 224.4.255.255` `233.252.0.0 - 233.255.255.255`) | Any enterprise can allocate a Multicast address from the transient groups for a global Multicast application and should release it when the application is no longer in use. Because these addresses are not permanently assigned to any application, they are called transient. Multicast addresses in the AD-HOC blocks were traditionally used for assignments for those applications that don’t fit in either the Local or Internetwork Control blocks. | ([View Highlight](https://read.readwise.io/read/01jnne60rvgbrhjraeyhc0ed5x)) --- ### IPv4 Permanent Multicast Groups The table below shows some of the well-known IPv4 Multicast addresses assigned to permanent groups by IANA. | Multicast Address | Usage | | ------------------------- | ----------------------------------------------------------------- | | 224.0.0.1 | All Multicast hosts in the subnet (All-hosts Multicast Group) | | 224.0.0.2 | All Multicast routers in the subnet (All-routers Multicast Group) | | 224.0.0.5 & 224.0.0.6 | All OSPF routers and All OSPF DRs (respectively) | | 224.0.0.9 | All RIPv2 routers | | 224.0.0.10 | All EIGRP routers | | 224.0.0.13 | All PIM routers | | 224.0.0.18 | VRRP | | 224.0.0.22 | IGMPv3 | | 224.0.0.102 | HSRP and GLBP | | 224.0.1.1 | NTP | | 224.0.1.39 & 224.0.1.40 | Cisco Auto-RP (RP-Announce & RP-Discovery) | | 224.0.1.129 – 224.0.1.132 | PTPv1/PTPv2 | ([View Highlight](https://read.readwise.io/read/01jnne9c0p79egsfbx4tn9vzsc)) --- ### Layer-2 Multicast Addressing (MAC addresses) A MAC address is 48 bits long and is a unique identifier for a Network Interface Card (NIC) on the LAN Segment. Also, MAC addresses are represented by a 12-digit hexadecimal number in the format `0012.3456.7890` or `00:12:34:56:78:90`. MAC addresses are essential for switching frames on Ethernet LAN segments. - In Unicast or Anycast transmission, ARP is used to resolve the destination’s MAC address. - In Broadcast transmission, the destination’s MAC address is `FF:FF:FF:FF:FF:FF`. - In Multicast transmission, IPv4 addresses are mapped to MAC addresses. ^103sb1 - The Multicast MAC address range is from `01:00:5E:00:00:00` to `01:00:5E:7F:FF:FF`. - The first 25 bits are fixed `(01:00:5E)`: `0000:0001:0000:0000:0101:1110:0xxx:xxxx:xxxx:xxxx:xxxx:xxxx`. - The last 23 bits are mapped from the Multicast destination group IPv4 address. - The first byte contains the individual/group (I/G) bit, which is set to 1 to indicate a Multicast MAC address. ([View Highlight](https://read.readwise.io/read/01jnneb2wjtyh53qsg7mwe75k0)) --- #### Mapping IPv4 Multicast Address to Multicast MAC Address - To map the IPv4 Multicast address to its corresponding MAC address, go through the following steps: ^glwaft 1. Convert the IP address to binary. Notice the first 4 bits are always `1110` for any Multicast IP address. 2. Replace the first 4 bits `1110` of the IP address with the six hexadecimal digits (or 24 bits) `01-00-5E`. 3. Replace the next 5 bits of the binary IP address with one binary `0` in the Multicast MAC address space. 4. Copy the last 23 bits of the binary IP address in the last 23-bit space of the Multicast MAC address. 5. Convert the last 24 bits of the Multicast MAC address from binary to six hexadecimal digits. 6. Combine the first six hexadecimal digits `01-00-5E` with the last six hexadecimal digits to form a complete multicast MAC address of 12 hex digits. ([View Highlight](https://read.readwise.io/read/01jnnebnkmkkkfb3c1fvtbs5j2)) --- #### Multicast Addressing MAC Overlap Note that in the last two examples (in the above video), one Multicast MAC address was mapped to multiple different Multicast IPv4 addresses: IP: `230.255.1.2` => MAC: `01:00:5E:7F:01:02` IP: `239.127.1.2` => MAC: `01:00:5E:7F:01:02` For a host, this overlap means that if its NIC card is programmed to listen to a particular multicast MAC address, and it could receive frames for multiple multicast groups. For example, imagine that a source is active on a LAN segment and is generating multicast group traffic to both `230.255.1.2` and `239.127.1.2`. Both groups are mapped to the same Multicast MAC address (`01:00:5E:7F:01:02`). If the host is interested only in packets for `230.255.1.2`, it can NOT differentiate the groups at Layer 2. All the frames are passed to a higher layer where the uninteresting frames get discarded, while the interesting frames are sent to the application for processing. So, Multiple IPv4 multicast group addresses can map to the same multicast MAC address. (32 IPv4 addresses > 1 MAC address; check the figure below). The 32:1 overlap must be considered when designing a multicast group addressing scheme. ([View Highlight](https://read.readwise.io/read/01jnnedqnek7mr76651q0y55rk)) ^gmpszf --- The last 23 bits in the multicast IP address must be unique in order to have one-to-one MAC address mapping. Also, it is recommended to avoid using groups in format `X.0.0.Y` and `X.128.0.Y` like `239.0.0.100` and `239.128.0.100`, because its multicast MAC addresses overlap with the Local Network Control Block `(224.0.0.X)`. ([View Highlight](https://read.readwise.io/read/01jnnef55jdjnpx1nwr7j2m309)) ^cfg3jk ---