IPV6 addresses

[Colin Law]

When I run ifconfig on an 18.04 machine which has been upgraded over
several versions I see an IPV6 address generated from the MAC address
of the form
fe80::xxxx:xxff:fexx:xxxx
which has been generated using the technique in
http://www.sput.nl/internet/ipv6/ll-mac.html.

On another machine with a fresh install, however, the fe80:: address
does not have any obvious relationship to the MAC address and has not
got ff:fe at the appropriate place. I deduce from this that there
other allowed techniques for generating the address but I can't find
any reference to them anywhere.  Can anyone elucidate?

Colin