Great trouble shooting and setup info here.
You will eventually find yourself trying to fix a network related problem which usually appears in one of two forms. The first is slow response times from the remote server, and the second is a complete lack of connectivity. These symptoms can be caused by:
 

 Why does the clock in my Linux guest consistently run more slowly or more quickly than real world time?
 

 This is the Intrusion Detection Cheat Sheet. A quick reference every Admin should have an eye on - if something unusual happens. I usually always have this PDF somewhere handy - and if it's only online...
 

 ARP is on nearly every LINUX box, and its a useful trouble shooting tool. However most admins never check the ARP table, just because it is not that common. However, sometimes, ARP is the answer to your issues. Check this little excursion to the ARP world !
 

 Provides an Interface to perform all kind of Lan/Network/Subnet/IP Address calculation and conversions.
 

 This is one of the best articles I have ever read on LINUX Network Troubleshooting. And it could be a tremendous help when you come across that diskless client that won't boot.
 

 Loads of links related to TCP/IP and other Unix related things
Excellent for learning networking and its ins-and-outs.
 

 Contains an excellent table on trouble shooting
It has some detailed explanations on the basics of TCP/IP and how to actually handle it.
 

 Usefull Links provided by the makers of Ethereal.
When troubleshooting, you will almost always at least once need information from links posted here.