MAC address to IPv6 link-local address

MAC address to IPv6 link-local address

The math

Link-local IPv6 addresses are used as part of the IPv6 network auto-configuration process. Instead of getting an address via DHCP, a NIC will hop on the network with a link-local IPv6 address and with this will have to ability to do further configuration automatically (soliciting neighbors, router, et cetera).

This link-local IPv6 is inferred from the NIC’s mac address.

A mac address is 48 bits, an IPv6 address is 128 bits. Here’s the conversion process step by step:

1. take the mac address: for example 52:74:f2:b1:a8:7f
2. throw ff:fe in the middle: 52:74:f2:ff:fe:b1:a8:7f
3. reformat to IPv6 notation 5274:f2ff:feb1:a87f
4. convert the first octet from hexadecimal to binary: 52 -> 01010010
5. invert the bit at position 6 (counting from 0): 01010010 -> 01010000
6. convert octet back to hexadecimal: 01010000 -> 50
7. replace first octet with newly calculated one: 5074:f2ff:feb1:a87f
8. prepend the link-local prefix: fe80::5074:f2ff:feb1:a87f
9. done!