Virtual Interfaces and Subnet Masks

Martin McCormick martin at
Fri Jun 24 11:33:37 GMT 2005

"Daniel Marsh" writes:
>Why don't you just setup an IP alias for the fxp0 interface?

	Thank you.  that both validates some of what I did, but raises
new questions.  Originally, I tried to bring up a second Ethernet card
which is on the system in question and which I thought we would use if
we ever had to.  This had worked once before when the dead system we
were replacing was on a different network and I just assumed it would
work on the same network.  Big mistake!

	After discovering the hard way about conflicting instances of
the same subnetmask and getting a bit nervous because we were still
down on the interface we needed most, I tried bringing up fxp1 as

ifconfig fxp1

This worked, but I soon realized that ifconfig gave it a default mask of which is wrong for that network.

	I then tried the alias for fxp0 exactly as you indicated and
got it but the packets going out still had a mask.  I
ended up, somehow, with that bad mask on both the primary interface
and the alias after trying to start over with ifconfig.

if config -alias
ifconfig fxp0 inet netmask
ifconfig fxp0 alias

I thought of bringing fxp0 down first but was doing all of this
remotely from home and was concerned that I'd loose the system
completely and it would be some minutes before one of us could
physically go there and take control.

	We or at least I don't do this very thing nearly often enough
because things under FreeBSD and Linux just don't break that often and
it is difficult to duplicate every possible permutation of trouble
meaning that sometimes, one gets nasty surprises.

	While not laughing at what happened too hard, can you think of
how I ended up with the bad mask that wouldn't go away?  Many thanks.

Martin McCormick WB5AGZ  Stillwater, OK 
OSU Information Technology Division Network Operations Group

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