Secondary IP Addresses

This is going to be a short post. I feel guilty for neglecting my blog (stupid ITIL), so I’m throwing this one together real quick.

There was a thread on Tech Exams recently, the poster was trying to figure out how to connect two subnets to a single Ethernet interface on a router. This was due to changing the address scheme within his company. Here’s what it looked like:

Very simple topology. The PC is part of the new address scheme and the printer is using a static IP from the old scheme. One of the easiest solutions here is to use a secondary IP on the Fa0/0 interface of R1. Here’s how it works:

EDGE(config)#int fa0/0
EDGE(config-if)#ip add 192.168.10.1 255.255.255.0
EDGE(config-if)#ip add 10.1.1.1 255.255.255.0 secondary

Very simple commands. Let’s verify:

interface FastEthernet0/0
 ip address 10.1.1.1 255.255.255.0 secondary
 ip address 192.168.10.1 255.255.255.0

It worked, the interface now has two IPs. Now we would just configure the IPs on the hosts (or DHCP server) using the proper subnet and gateway. Once the hosts are configured, everything will work as desired.

Topic Ideas…

As I explained awhile back, I’m working on the ITILv3 cert and haven’t really been digging into any Cisco/CCIE material. I’m not sure how much longer I’ll be at this ITIL stuff, but I really hate not posting. Without studying, I don’t have a ton of ideas for posts. I have one I’m doing later today on Secondary IP Addresses, but it’s short and simple.

Please post some article ideas in the comments section if you have any. Anything you guys would like to see, let me know. I can’t promise that I’ll do all of them immediately, but I will keep them on the backburner and post when I have time to throw something together or learn the topic well enough to get something decent up.

Go!