Re: Subnetting

Stuart Levy (
Tue, 10 May 88 22:51:40 CDT

It looks as though 4.3BSD subnetting code doesn't store a net mask per route.
There's a net mask per -interface-, but it doesn't seem to be used the
way you'd hope. There's effectively just one subnet mask per net for
routing purposes (I think it uses the mask set by the first interface on
the net) -- that is, 4.3BSD silently assumes equal-sized subnets on a
given net. If I say

        ifconfig ex0 netmask
        ifconfig ex1 netmask

the interface route for ex1 has a destination address of ""
(reasonable) BUT the route is only used when sending to 128.101.16.anything.
Sending to 128.101.17.anything, ..., 128.101.31.anything does not increment
the interface route's usage count.

4.3's multiple subnet masks are meaningful for interfaces on different nets
but not for variable-sized subnets on a single net. It'll be interesting to
see whether SUN 4.0 works the same way.

This archive was generated by hypermail 2.0b3 on Thu Mar 09 2000 - 14:42:13 GMT