Oct 21 2009
Layer 2 Multicast Addresses
Network Interface cards can accept Ethernet frames destined towards the following MAC addresses.
- BIA (Mac Address of the NIC)
- 0xFFFF.FFFF.FFFF (Broadcast Address)
- A range of multicast Address
In this post we will discuss the range of multicast addresses.
Ethernet Multicast Addresses
An Ethernet multicast address consists of the
- The IANA owned OUI MAC address 01:00:5e (24 bits) – which includes a multicast bit.
- IANA owns the OUI MAC address 01:00:5e, therefore multicast packets are delivered by using the Ethernet MAC address range 01:00:5e:00:00:00 – 01:00:5e:7f:ff:ff.
- The lower 23 bits of the multicast IP address are mapped into the remaining 23 bits of available ethernet address space.
The multicast addresses are in the range is 224.0.0.0 through 239.255.255.255. Therefore we can conclude the following:
- All multicast addresses have the first four bits in the first octet set to 1110.
- Therefore 28 bits remaining in the IP address to identify a unique multicast address.
The Diagram below illustrates the multicast MAC address range as well as an example of a Multicast Layer 3 Address encoding.
Multicast IP Address Encoding
When encoding layer 3 multicast addresses, the following observations can be seen.
- The first octect In the layer 3 address ie 239 in our example is not encoded.
- As all layer multicast addresses begin with 1110 as the first octect.
- As the remaining four bits in the 1st octet of the multicast IP address are not encoded, we can say that 4 configurable bits are lost.
- We can also note that as the 4th Octet in the Multicast MAC address has an upper limit of 7F, only 7 bits are available to encode the 2nd octet of the multicast IP addresses which means an additional configurable bit is lost in encoding.
- Therefore a total of 5 configurable bits are lost during multicast IP to MAC encoding.
- The net result here is that 32 unique Multicast IP addresses when encoded will result in the same Multicast MAC address.
The diagram below illustrates how 32 Multicast Layer 3 addresses are encoded into 1 Multicast MAC Address.
OSI Model and Multicast/Unicast - Untangle Forums
Feb 20, 2011 @ 19:58:32
[...] Now the first two and the last one can happen at either layer 2 or layer 3. Layer 2 multicast : http://www.debugall.co.uk/2009/10/21…ast-addresses/ Layer 3 multicast : http://www.cisco.com/en/US/docs/inte…k/IP-Multi.pdf Layer 2 broadcast : [...]