use of non-contiguous masks in address lookups ?

[sthaug at nethelp.no]

> every now and then the issue comes up on whether we still need
> to support non-contiguous masks in address lookups.
> I seem to remember someone (perhaps on this list) making a
> case for their presence, but forgot the details.
> So, does anyone know of a practical use of non contiguous masks ?

I vote for removing non-contiguous masks. They are incompatible with
CIDR, which was introduced in 1993 (!).

Non-contiguous masks have been unsupported in many routers produced
the last 10 years or so.

E.g. for Juniper M/MX/T routers you specify an interface address with
a contiguous mask (1.2.3.4/29 etc). It is *not possible* to specify a
non-contiguous mask. Address lookup is always based on longest prefix
match.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no