LateNightHacking Louis Journal

Auth

Note to self: There are smaller things than class C networks nowdays. I think we've finally traced down a crazy network connectivity problem we've been having at work. We have some verdor servers that are kept on an isolated network. We have two servers that specially set up to be the only ones to talk to the vendor servers. These custom servers are dual homed, with a NIC in the isolated network and a NIC in the main network. Now, we were trying to connect to the custom servers from an external client, and we were getting weird intermittent connectivity and having problems establishing a connection. Well, we looked at the external client's IP and it was on the same subnet as the isolated vendor servers! Or so we thought. The routing table on the custom server was set up to send packets addressed to the external clients to the isolated network. No wonder there were problems.

We talked to our network guy to find out what the heck was going on. We wanted to know why this external client was on the same subnet as the isolated servers. He told us that they were using smaller networks! The network mask for the isolated network was 28 bit, not 24 bit (class C) like we had assumed when configuring the custom server. Finally, it starts making sense. Hopefully we can get the routing table on the custom server after market hours today and everything will just start workign correctly!

In case you didn't follow that, an IP address is 32 bits long. For routing purposes, the address is broken into two parts, the "where" part and the "who" part. However, where that split occurs is variable. The "where" part identifies which subnet, and the "who" part identifies which computer on the subnet. Often, 24 bits of "where" and 8 bits of "who" are used. This is convenient because when an IP is represented in standard A.B.C.D form, A.B.C is the where and D is who. In our case, there are 28 bits of "where", which means that part of D is where and part is who. (The advantage is that this allows you to have more wheres, each with fewer whos.) Because we had the routing table misconfigured, packets were getting set the wrong direction and never reaching where they were supposed to be going.

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What a crazy morning. Price problems (that routing thing), weird GUI stutters, a GUI crash (looks like another Formula 1 issue), a trader starting multiple instances of his GUI, and Comstock sending us weird price messages. I thought it might be a bad idea to schedule three phone interviews on one morning.