Back at one of my favorite EDUs, I was brought in to consult on some switch configuration for a new Radio over IP (RoIP) system for the campus police. They of course contracted the low bidder to install the system, which amounts to a million dollar investment. However, due to being A) radio guys and B) low bidder, the contractor decides to cut corners on what they feel is the least important part of the system: the switch that sits between the RoIP servers and the rest of the network. Yes, a million dollar project connected to the LAN/WAN through $100 HP Procurve switches.
So the contractor has been giving my customer grief for their lack of assistance in helping get their servers on the network. My customer provided exactly what they asked for: 2 network ports in two different VLANs and IP information for subnets that could route between sites. The VLANs are configured, the networks routed, and one of the customer techs plugs in a laptop to prove that he can reach the rest of the network. No one here has any experience with HP, so they show me the config. I spot the problem in about 5 seconds – the radio guys had the uplinks looking for tagged packets.
We contact the radio guys, tell them what they need to do to fix it, and the customer sends techs out to assist. They reconfigure the switch according to my recommendations and magically the two laptops at two different sites can now talk to each other. Plug in the radio servers, nothing. Nada. The customer tech reviews the server configuration and gets to the netmask. “Uh guys, why is your netmask 255.0.0.0? We told you 255.255.255.0″. “Oh, we wanted to reach the servers at all the sites so we used a bigger netmask” facepalm
See, the customer had provided /24 networks at each site, 10.X.194.0/24, where X is the site number. Due to their utter lack of IP networking knowledge, the radio guys assumed that they would have to set the netmask to 255.0.0.0 so that servers on 10.1.194.0 could talk to servers on 10.2.194.0 and so on. They also never mentioned that they decided to change this vital information in the previous TWO WEEKS of troubleshooting.
TL;DR You can’t IP freely by changing the netmask.