How to specify target host in Access-list on 1700 router

Question

I want to be able to specify the target host on an access list and when I try to enter the IP and sub-net mask I get wierd result. This is on a 1700 router. I type: access-list 100 permit tcp any XXX.XXX.XXX.XXX 255.255.255.248 eq smtp where XXX.XXX.XXX.XXX is a public IP of a virtual email server on my inside.
I get:
access-list 100 permit tcp any 0.0.0.2 255.255.255.248 eq smtp
Why does XXX.XXX.XXX.XXX get interpreted as 0.0.0.2?
Thanks,
Dave

Answer 1

Dave,
The address got converted to 0.0.0.2 because you used a subnet mask (255.255.255.248) where you should have used a wildcard mask (0.0.0.7).
Regardless of what the network portion of the address was, when the router sees "255" in any position in the wildcard mask, it interprets that as "it really doesn't matter what number is in this part of the IP address". So it corrects your notation and replaces that part of the IP address with the placeholder "0".
The fact that it put a ".2" at the end of the address indicates that the binary pattern of whatever XXX.XXX.XXX.XXX was ended in "010". The last octet was one of the numbers in this sequence: .2, .10, .18, ... (increments of 8), .114, or .122. The "248" in the last part of your wildcard mask told the router "it doesn't matter what number's here, as long as the last three binary bits match". The router just simplified the last .XXX you entered to the smallest number that had a matching binary pattern; in this case it was ".2".
Something to remember: Use subnet masks for static routes and interface addressing; and wildcard masks for ACLs.
The easiest way to calculate the wildcard mask you want, if you're used to seeing things in subnet mask format, is to subtract the subnet mask from 255.255.255.255. For example:
255.255.255.255
-255.255.255.248 (subnet mask)
0.0.0.7 (wildcard mask)
If you want to specify a single host address rather than a masked range of addresses, use the notation "host XXX.XXX.XXX.XXX". If you use the notation "XXX.XXX.XXX.XXX 0.0.0.0" where 0.0.0.0 is the wildcard mask, the router will convert it to "host XXX.XXX.XXX.XXX". (Go ahead, try it and see.)
Similarly, if you want to specify all host addresses, use "any" as you have already done; or you can try "0.0.0.0 255.255.255.255" and the router will convert it to "any" for you. (Try this one too.)
Check out the useful IP Subnet Calculator download at http://www.Boson.com -- it's free:
Wildcard Mask Checker & Decimal-to-IP Calculator
a neat little utility to check what your wildcard mask actually matches, and, converts from Decimal to IP address format.
http://www.boson.com/promo/utilities.htm
Hope this helps.