In message <200006281346.NAA13366@orchard.arlington.ma.us>, Bill Sommerfeld wri
tes:
>> I suppose it's off by default to avoid leaking network information to
>> a potential attacker.
>
>If this is a concern, we could change it to send the replies with
>TTL=1; anyone already on the wire can make a good guess at what the
>netmask is..

It should be off by default because otherwise, one misconfigured 
machine can screw up an entire network.  The rationale, I would say, is 
the same as ensuring that a random multi-homed machine doesn't act as a 
router.

		--Steve Bellovin