[rescue] Re: [offtopic] Cisco 2501 and "subnet mask bug"?

[Robert Novak]

On Sat, 4 May 2002, Patrick Giagnocavo wrote:

> I have a weird situation where I borrowed a 2501 for a month from
> someone and put it into service on a T1 line. With it are 32 IPs.
> 
> The weird part is that (apparently) no one not on the same subnet can
> reach any of the machines IF the first part of the IP address starts
> with the same number.

Sounds like you're using CIDR but your subnet mask isn't set correctly. I
have this problem with my /25 subnet sometimes, which is carved out of a
class A subnet... at least until I remember to edit /etc/netmasks or the
like. 

[I'm changing your third decimal to 17 to avoid confusion with the 32
addresses vs .32.x subnet in the following examples]

If you have 172.16.17.0/27 (32 IP addresses), your subnet mask should be
255.255.255.224. This tells your system to route anything not in
172.16.17.0-172.16.17.31 through your default route.

Without this, if it's 172.16.17.0 the default subnet mask is 255.255.0.0,
so your machine would try to send traffic to 172.16.*.* (NOT
172.*.*.*--172 is a class B) to the local wire network.

If you had a real network of, say, 64.80.151.64-64.80.151.95, which is
carved from a class A /8 network, the issue would be with 64.*.*.* unless
you have the right netmask. If you had 198.70.36.64-198.70.36.95, which is
carved from a class C /24 network, the issue would be with 198.70.36.*.

You might want to go to
http://www.telusplanet.net/public/sparkman/netcalc.htm with your real
address and subnet info, and play with the calculator there. It will tell
you what your netmask and pertinent numbers would be. 

Hope this helps,
Rob
 

Robert Novak, Indyramp Consulting * rnovak at indyramp.com * indyramp.com/~rnovak
	"I don't want to doubt you, Know everything about you
      I don't want to sit Across the table from you Wishing I could run."