Multiple Subnets with a single gateway address?

[ryan.vaughn]

We have gotten a request to set up a router interface with multiple networks on a single interface, however they only want to use 1 interface IP address as to not lose an IP address in each subnet for the gateway. The user has claimed it has been done for them before, but I do not see how it could work. We use OSPF for our internal routing, so in the past we just apply the IP address to the interface and it gets entered into the OSPF and we do not have to static route.

We could static route the IP's to the interface, but you need an address to route to. AFAIK, this is not possible:

Int E3/0

ip address 10.10.10.1 255.255.255.0

ip route 10.10.11.0 255.255.255.0 10.10.10.1

ip route 10.10.12.0 255.255.255.0 10.10.10.1

ip route 10.10.13.0 255.255.255.0 10.10.10.1

You cannot route IP's to an address on the router interface. But this is what they are looking to do. I have had someone mention numberless routing but I have no experience with this. Has anybody run into situations like this in the past?

Thanks for the help

-Ryan

[ryan.vaughn]

To followup with my own message...

This was new to me, I have never done anything similar to it. Somebody following this thread may find it useful.

This is what I was trying to do, and it is a working config:

Interface E3/0

ip address 10.10.10.1 255.255.255.0

ip route 10.10.20.0 255.255.255.0 E3/0

ip route 10.10.30.0 255.255.255.0 E3/0

With this config you can specify a single "gateway" address on a network segment and route multiple networks to that interface. This way you are not losing 1 IP from every block as the gateway address for the network. I believe earlier versions of MS Windows did not allow you to apply a gateway address not in the same network as the ip address of the machine. But this will work well if you use later versions of windows/unix or any other OS that supports it.

-Ryan

[mdgregg]

1. Is there a problem with setting the IP for the E0 as: ip address 10.10.8.1 255.255.248.0 ?

2. You may have to assign VLANs on the switches to reduce broadcasts.

3. I've also seen the use of a BVI interface on the router to assign different IPs for managability of switches seperately from the subnets.

[jimb]

It depends on the nodes. If you are talking about Windows clients for example, you can point them back at your router's Ethernet Interface IP with no problem. MAC on the other hand does not like IPs from different subnets.

If you are pointing to subnets behind other routers that reside on the same ethernet segment, point each route to that interface.

i.e."

ip route 192.168.1.0 255.255.255.0 fastethernet0/0