Hi, I read your posting and I do hope this will help you out. If you use the IP address from your example 81.91.81.1 /20 you have 4096 usable subnets and 4094 usable host per subnet, to me that is alot of hosts and unless you have a hugh network you will not need them all. Looks to me you want to use a /20 subnet mask on fa0/0 and a /24 on fa1/0 and fa1/1 do some research into IP classless command. Check out the link below I think it will help.
auto-summary is a similar concept, but is used for routing, where the
routing protocol would "auto-summarize" the routes using the above obsolete
mechanism. You should also enable "no auto-summary" as well
From Cisco
IP Classless
Where the ip classless configuration command falls within the routing and forwarding processes is often confusing. In reality, IP classless only affects the operation of the forwarding processes in IOS; it doesn't affect the way the routing table is built. If IP classless isn't configured (using the no ip classless command), the router won't forward packets to supernets. As an example, let's again place three routes in the routing table and route packets through the router.
Note: If the supernet or default route is learned via IS-IS or OSPF, the no ip classless configuration command is ignored. In this case, packet switching behavior works as though ip classless were configured.
router# show ip route
....
172.30.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 172.30.32.0/20 [90/4879540] via 10.1.1.2
D 172.30.32.0/24 [90/25789217] via 10.1.1.1
S* 0.0.0.0/0 [1/0] via 10.1.1.3
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml
