The Area 0

Today we ran into an issue with our BGP advertisements. To give you a little history, we were allocated 198.49.81.0/24 and 198.49.82.0/24 back in 1993, which was pre-CIDR. (I bet you can see where this is going, already.)

We have been using 198.49.81.0/24 for a number of years, obviously longer then I’ve been at PSU. I was told that we’d never used the “other half” of our assigned IP space. So basically, I knew:

• we had 198.49.81.0/24
• that it was only half of the address space

I made an assumption about what address the other half was, and earlier this summer I had asked our ISP:

“Should we not be using 198.49.80.0/23?” I did the math a couple times. I checked ipcalc, I knew it was correct. So not realizing that the assumption I had made was a poor one, we went ahead and advertised 198.49.80.0/23 via BGP. The ISP didn’t catch it, either. A simple oversight. I suppose they figured we knew what address space we owned….

In the days of CIDR, of course we would be given the /23! It would be just silly not to. In fact, 198.49.80.0/24 is unallocated. It was a simple case of not realizing that the addresses were pre-CIDR, and therefore not contained by a contiguous bit boundary. OOPS.

It wasn’t a big deal to fix it, we simply added the two correct prefixes to our BGP advertisements, and our ISP simply filtered the /23 we were also advertising. There is some more work to be done upstream, I am told, but everything is working as it should at this point.

Props to Cort at KanREN for spotting the issue, and helping us get it resolved.