About interface id in aggregatable global unicast address

[Matt Crawford]

> I think the length is too long and waste address space. (The field
> make the network part of IPv6 address to less than 64 bits) Since
> the requirement of the field is to make the interface on a link
> unique, why we use other method, such as EUI-48 only (only as an
> example), for the field?

1. Some media do not have a 48 bit identifier, but a 64-bit one, and
an IEEE-defined mapping exists from 48 to 64 bit identifiers.

2. Some router implementors have claimed that routing on 64 bits
rather than 80 can be significantly faster.

3. The door is left open by this choice to some future scheme in
which the interface identifier is a globally unique identifier for a
node.