If you have anything to do with network administration and/or security,
you have to be well grounded in in DNS theory. It's the service that
most everything else on the Internet depends on. It's also the source
of many of your network problems, intentional or otherwise. Here's a paper by Gideon T. Rasmussen which discusses basic troubleshooting steps. It's a bit CyberGuard-centric but does give you an idea for starting points for troubleshooting problems.