Microsoft Knowledge Base Article 301989 contains the following summary:
This step-by-step article describes how to capture Wide Area Network (WAN) traffic with Network Monitor.
While collecting information from the network's data stream, Network Monitor displays the following types of information:
• The source address of the computer that sent a frame onto the network. (This address is a unique hexadecimal (or base-16) number that identifies that computer on the network.)
• The destination address of the computer that received the frame.
• The protocols used to send the frame.
• The data or a portion of the message being sent.