addresses from multiple subnets over the same Router LAN port (Cisco ISR 2951)

[kevin.mcguire]

Hi,

I'm in the process of converting our class C network to a Class A network to allow more client devices to connect to the internet.  Because I have limited time to actually work on the routers and switches, I'm looking to see if I can put together a temporary hybrid network.

Is there any way I can configure one of the LAN ports on my 2951 router to allow traffic from both the new Class A network (10.x.x.x) along with traffic from the current class C (192.x.x.x).  There are several devices currently on the 192.x.x.x subnet with Static IP addresses.  These devices will be updated to new Class A static IP adrresses, but I may not be given the time to change both the router's ports and the statically addressed client devices in the same work session.

Thanks in advance for any suggestions.

[Richard Burts · Accepted solution]

There are probably some things about your environment that we do not yet know which might affect the suggestions that we make. But based on your description so far I believe that the solution that you are looking for is to use secondary addressing. This allows two (or perhaps more than two) subnets to exist on the same router interface. It might look something like this

interface FastEth0/0

ip address 192.168.12.1 255.255.255.0

ip address 10.16.0.1 255.255.0.0 secondary

Now both subnets will work. The existing hosts in the 192.168.12 network will continue to work with their existing gateway of 192.168.12.1. And you can add new devices in 10.16 and can change hosts that were in the 192.168.12 network. You would change the device IP address, mask, and gateway of 10.16.0.1. Devices in both subnets would be able to communicate with each other (some may need to send this traffic to their gateway and have the router forward it to the destination (which is connected on the same router interface).

HTH

Rick

[kevin.mcguire]

Thanks, Rick. Had forgotten about secondary addressing from my CCNA training a few years back.

[Richard Burts]

I am glad that my suggestion was able to point you in the right direction. Some of those details do tend to slip away over time. Glad I could remind you of it. Thanks for marking this question as solved. This will make it easier for readers in the forum to identify discussions that have helpful information.

HTH

Rick