All Platforms:

DNS - How It Works

Ethereal - Free Network Monitor & Packet Sniffer

Networking Primer - from Novell

RFC Editor.com - useful networking protocol info

Speed Test - test your Internet connection

Traceroute (UC Berkeley UNIX)

Windows:

Create PPPoE Internet connection in Windows XP Pro

How To Diagnose and Test TCP/IP or NetBIOS Connections in Windows 2000

Maintaining Windows XP

Nbtstat

Nbtstat reveals who is logged on

NetBIOS Service codes in the NetBIOS suffix

Troubleshoot Connectivity Issues

Troubleshooting the Microsoft Computer Browser Service

Troubleshooting Tools and Strategies

Windows 2000 DNS