"Gerald Combs" <gerald@zing.org> wrote:

> >>It is?  Subnet masks are nearly always a string of ones, followed by a
> >>string of zeroes, e.g. "255.255.255.0".  The ones indicate which bits in
> >
> >
> >   'Usually'?  When are they not?

> AFAIK, the convention of a contiguous set of ones followed by a
> contiguous set of zeroes isn't explictly mandated, but it makes routing
> not only easy, but possible.  Which mask indicates the best path:
> 255.255.255.0 or 255.255.0.128?

  Looking back at RFC 950, non-contiguous masks were even explicitly
allowed, but it's clear that noncontiguous masks break things like routing.
And isn't routing what puts the INTER in INTERnet?