How do I... Monitor traffic on a network for free with Ethereal
Source: David Davis
Network administrators have to know what is going within the network at all times. This task requires a monitoring application that can be relied on. If you want to monitor traffic on your network, you can purchase a packet analyzer. However if your budget is tight, you don't have to spend any money at all. Here's how you can do so using Ethereal, which is an open source protocol analyzer available for a variety of operating systems. This How do I... download shows you how to install Ethereal and how to use it.
This download is also available as a TechRepublic article.