Can I use the same subnet on 3 routers?

[matthew-gallagher]

We currently have 2 different networks which connect at 1 main router. There are 4 off campus sites with 2 2621 routers at each coming back to 1 main router via frame relay. Currently the main router is connected to a Cisco 1005 which connects to the Internet. We want to segment the two networks so there will be a 2nd 2621 going in at main campus. The frame is being replaced by Point to Point T1's ( 4 T1 DSU's in each main campus router). The 2 F0/0 will be connected to their respective networks and F0/1we will attach to a switch along with the Internet routers E0 interface because there is only 1 ISP at this time. Can these 3 interfaces be part of the same subnet

( from a Class C, subnet 224 ) or should we just use dummy IP's?

[rsissons]

The only way you can have more than one network segment in the same subnet and still continue to route IP on other interfaces is to configure the routers for IRB, Integrated Routing and Bridging.

This is covered in the URL -

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/ibm_c/bcprt1/bctb.htm

It is not however a very elegant or scalable solution. It is better to route IP wherever possible which means that every network segment must have a unique network address.