Routing Help Please

[Steven Chen_1]

Hi,

We are going to change IP scope from 207.186.218.1/24 to 172.16.16.1/28 in the whole companay. On one server we cannot change IP address for its too old. We plan to put a router in front it and route a network segment of 207.186.218.1 to the router. Thus we don't have to change its IP and all clients with the new IP addresses of 172.16.16.1/28 still can use it.

I don't know how to implement it. I can make it that far as to have clients to connect still use 207.186.218.1/24 IP and connect to this router.

What is the way to have it work, i.e.
1) that server IP not change
2) client with changed IP can connect

I am debating with manager, and are beat with this.

Very appreciated if giving any help!!

Steven

[Kevin Wright · Accepted solution]

It's amazing to me that you can't change the server's IP..why not? You could just use your regular router, the default route of your clients, and put a static route to the server.

[Ron Kinner]

If you have a Cisco router then you just use a secondary address on it to handle the odd ball. The router will happily switch between the two addresses on the same interface and you won't have to waste an interface for the purpose. You may want to add

ip route-cache same-interface

to the config under the interface. That will make it faster at switching between the two.

Let's say your old box has a default gateway of 207.186.218.254 and you don't want to change anything on the old box. You assign E0/1 as the interface for the 172.16.16 network and you want it to be 172.16.16.1 (which will be the new default gateway for the new subnet). You plug your switch into this interface and all of your devices including the odd ball are plugged into the switch.

Then you do
conf t
ip subnet-zero
ip classless
int e0/1
ip add 172.16.16.1 255.255.255.240
ip add 207.186.218.254 255.255.255.0 second
ip route-cache same-interface
end
wr me

Now the router will switch between the two subnets without a problem and as an advantage you do not have to recable the devices. They will still be able to talk using their old IP address until you get around to changing them. (Ip classless and IP subnet-zero are on by default in 12.0 and higher code so you don't need them if you are on a new router.)

Of course, you can use a router with 2 interfaces and put each subnet on a separate interface but then you have to do some recabling to get it to work.

Ron