Multicast IP to MAC Address Conversion

The OUI (1st 24 bits of the MAC address / 1st 3 octets of the MAC address) is common for any multicast IP address. They are 01-00-5E. So the next 24 bits of the MAC address can be varied but actually we take only the last 23 bits from the IP address to replace the last 24 bits of the MAC address. So the 1st starting bit of the 4th octet is always 0..

01 - 00 - 5E - 0000 0000 - 0000 0000 - 0000 0000

Because we always fix the 1st bit to be 0 of the last 24 bits taken from the IP, there are 32 IP addresses for a given MAC address though there is only one MAC address for a given IP address. See the examples..

IP to MAC Conversion Example

Find the MAC address of 226.139.1.2

Take the last 3 octets and convert into binary (139.1.2)

1000 1011 . 0000 0001 . 0000 0010

Ignore the 1st bit and combine the 1st 25 bits of common MAC and last 23 bits of given IP..

01 - 00 - 5E - 0000 1011 . 0000 0001 . 0000 0010

which is --> 01-00-5E-0B-01-02


MAC to IP Conversion Example

Find the IP address of 01-00-5E-0B-01-02

Take the last 3 octets and convert into binary (OB-01-02)

0000 1011 . 0000 0001 . 0000 0010 -> 11.1.2

so the last 23 bits can be came from the following banaries too.

1000 1011 . 0000 0001 . 0000 0010 -> 139.1.2

So 16 IP address can be written from 11.1.2 and another 16 can be written from 139.1.2 as the last 3 octets of the IP.

224.11.1.2 - 239.11.1.2
224.139.1.2 - 239.139.1.2

So following are all the addresses which can be mapped to 01-00-5E-0B-01-02

224.11.1.2	224.139.1.2
225.11.1.2	225.139.1.2
226.11.1.2	226.139.1.2
227.11.1.2	227.139.1.2
228.11.1.2	228.139.1.2
229.11.1.2	229.139.1.2
230.11.1.2	230.139.1.2
231.11.1.2	231.139.1.2
232.11.1.2	232.139.1.2
233.11.1.2	233.139.1.2
234.11.1.2	234.139.1.2
235.11.1.2	235.139.1.2
236.11.1.2	236.139.1.2
237.11.1.2	237.139.1.2
238.11.1.2	238.139.1.2
239.11.1.2	239.139.1.2