Multicast L2 Addressing
An L2 multicast address is 6 bytes, or 48 bits.
The address is built by combining these three things.
Multicast Prefix + 0 + low-order-23-bits-from-v4
Multicast Prefix
The Multicast Prefix is 01:00:5E 3 bytes or 24 bits.
25th Bit
A binary zero 0
Last 23 bits
Using 232.10.10.10
Copy the low order 23 bits directly from the v4 address.
232.10.10.10/8
1110 1000 . 0000 1010 . 0000 1010 . 0000 1010
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000 1010 0000 1010 0000 1010
Building the Address
Using 232.10.10.10
1 : 0 : 5E : 0A : 0A : 0A
0000 0001 . 0000 0000 . 0101 1110 . 0000 1010 . 0000 1010 . 0000 1010
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Multicast Prefix | The 23 bits from the v4 IP
Always 01:00:5E |
└─ The required zero bit for multicast
becomes 01:00:5E:0A:0A:0A
Multicast Overlap
To express a v4 multicast address uniquely in L2 we need 28 bits, since the first 4 bits of every v4 multicast address are 1110
L2 in multicast only gets 23 bits, which means 5 bits of overlap.
2^5 is 32, so there are 32 v4 addresses that share the same L2 address.
All 32 IPv4 addresses mapping to 01:00:5E:0A:0A:0A
══════════════════════════════════════════════════════════════════════════════
Address Octet 1 Octet 2 Octet 3 Octet 4
──────────────────────────────────────────────────────────────────────────────
224. 10.10.10 1110 0000 0000 1010 0000 1010 0000 1010
224.138.10.10 1110 0000 1000 1010 0000 1010 0000 1010
225. 10.10.10 1110 0001 0000 1010 0000 1010 0000 1010
225.138.10.10 1110 0001 1000 1010 0000 1010 0000 1010
226 .10.10.10 1110 0010 0000 1010 0000 1010 0000 1010
226.138.10.10 1110 0010 1000 1010 0000 1010 0000 1010
227 .10.10.10 1110 0011 0000 1010 0000 1010 0000 1010
227.138.10.10 1110 0011 1000 1010 0000 1010 0000 1010
228 .10.10.10 1110 0100 0000 1010 0000 1010 0000 1010
228.138.10.10 1110 0100 1000 1010 0000 1010 0000 1010
229 .10.10.10 1110 0101 0000 1010 0000 1010 0000 1010
229.138.10.10 1110 0101 1000 1010 0000 1010 0000 1010
230 .10.10.10 1110 0110 0000 1010 0000 1010 0000 1010
230.138.10.10 1110 0110 1000 1010 0000 1010 0000 1010
231 .10.10.10 1110 0111 0000 1010 0000 1010 0000 1010
231.138.10.10 1110 0111 1000 1010 0000 1010 0000 1010
232 .10.10.10 1110 1000 0000 1010 0000 1010 0000 1010 < --- This is our SSM address
232.138.10.10 1110 1000 1000 1010 0000 1010 0000 1010
233 .10.10.10 1110 1001 0000 1010 0000 1010 0000 1010 < --- An address in the GLOP block
233.138.10.10 1110 1001 1000 1010 0000 1010 0000 1010
234 .10.10.10 1110 1010 0000 1010 0000 1010 0000 1010
234.138.10.10 1110 1010 1000 1010 0000 1010 0000 1010
235 .10.10.10 1110 1011 0000 1010 0000 1010 0000 1010
235.138.10.10 1110 1011 1000 1010 0000 1010 0000 1010
236 .10.10.10 1110 1100 0000 1010 0000 1010 0000 1010
236.138.10.10 1110 1100 1000 1010 0000 1010 0000 1010
237 .10.10.10 1110 1101 0000 1010 0000 1010 0000 1010
237.138.10.10 1110 1101 1000 1010 0000 1010 0000 1010
238 .10.10.10 1110 1110 0000 1010 0000 1010 0000 1010
238.138.10.10 1110 1110 1000 1010 0000 1010 0000 1010
239 .10.10.10 1110 1111 0000 1010 0000 1010 0000 1010
239.138.10.10 1110 1111 1000 1010 0000 1010 0000 1010 < --- an Organizational scope address
══════════════════════════════════════════════════════════════════════════════
^^^^ ^
|||| |
└└└└──└─ I incremented these five bits to show the pattern.