The purpose of this article is to provide you with the information needed to capture network traffic from a local area network using Microsoft's Network Monitor. The text of this article comes directly from the Network Monitor's Help file and should be referenced for more detailed instructions.

Network Monitor is a network diagnostic tool that monitors local area networks and provides a graphical display of network statistics. Network administrators can use these statistics to perform routine trouble- shooting tasks, such as locating a server that is down, or that is receiving a disproportionate number of work requests. 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.

The process by which Network Monitor collects this information is called capturing. By default, Network Monitor gathers statistics on all the frames it detects on the network into a capture buffer, which is a reserved storage area in memory. To capture statistics on only a specific subset of frames, you can single out these frames by designing a capture filter. When you have finished capturing information, you can design a display filter to specify how much of the information that you have captured will be displayed in Network Monitor's Frame Viewer window.

To use Network Monitor, your computer must have a network card that supports promiscuous mode. If you are using Network Monitor on a remote machine, the local workstation does not need a network adapter card that supports promiscuous mode, but the remote computer does.

To capture across networks, or to preserve local resources, use the Network Monitor Agent to capture information using a remote Windows NT computer. When you capture remotely, the Network Monitor Agent gathers statistics from a remote computer, and then sends these statistics to your local computer, where they are displayed in a local Network Monitor window.

Once data has been captured either locally or remotely, the data can be saved to a text or a capture file, and can be opened and examined at a later time.

Note The core functionality of Network Monitor, described in Help, is supported by Microsoft Product Support Services. Network-dependent tasks, such as interpreting data that you capture from your network, are not supported. The Network Monitor Agent is supported for Windows NT, but is unsupported on Windows 3.1 and Windows for Workgroups workstations.

Creating an address list

To use address pairs in a Capture filter, you should first build an address database. Once this database is built, you can use the addresses listed in the database to specify address pairs in a capture filter.

To create an address list, follow these steps:

1.	From the Capture menu, select Start. Optionally, open a .cap file in the Frame Viewer window.
2.	When you have finished capturing, select Stop and View from the Capture menu to display the Frame Viewer window.
3.	From the Display menu, select Find All Names. Network Monitor processes the frames, then adds them to the address database.
4.	Close the Frame Viewer window, and display the Capture window.
5.	From the Capture menu, select Filter to display the Capture filter dialog box.
6.	In the Capture Filter dialog box, double-click on the Address Pairs line. Or, choose Address in the Add groupbox.

Network Monitor displays the address database you have created. You can use the names in this database to specify address pairs in the Capture filter.

Capturing data between two computers

To monitor traffic between two computers, follow these steps:

1.	From the Capture menu, choose Filter to display the Capture Filter dialog box.
2.	Double-click on the ANY<->ANY line to display the Address Expression dialog box.
3.	In the left window of the Address Expression dialog box, select the address of a computer.
4.	In the right window of the Address Expression dialog box, select the address of a computer.

When you have done this, choose the Next button at the top of this window for more instructions.

1.	In the Direction window, of the dialog box, choose one of the symbols: • Choose the <--> symbol to monitor the traffic that passes in either direction between the addresses that you have selected. • Choose the --> symbol to monitor only the traffic that passes from the address selected in the left window to the address selected in the right window.
2.	Choose OK.
3.	In the Capture Filter dialog box, choose OK.
4.	From the Capture menu, choose Start.

Saving captured data

Use the Save As command to save capture statistics to a capture file or to save changes to capture files that you have modified. Later, to view frames saved to file, you can open this file and display the statistics in Network Monitor's Frame Viewer window.

To save the captured frames to a capture file or text file:

1.	Do one of the following: • On the Toolbar, click the File Save button. -or- • From the File menu, choose Save As.
2.	Do one the following: • To save the file to the current drive and directory, in the File Name box, specify a file name and an extension. If you are saving a file that you have modified, you cannot save it under its original name in the same directory. • To save the file to a network share to which you are not connected, choose the Network button, and then use the Connect Network Drive dialog box to establish the connection. • To save the file to a different drive or directory, do the following: a. In the Drives box, select a new drive. b. In the Directories box, select a new directory. c. Type the file name.
3.	To save only those frame statistics that meet the specifications of the current display filter, choose Filtered. This option is available only if you are saving data from the Frame Viewer window.
4.	To save a particular range of frames, type the beginning and ending frame numbers in the From and To boxes.
5.	Choose OK.

Note When a range of frames is saved to a capture file, the numbers associated with the frames are changed; in a capture file, frame numbers always begin with 1, regardless of the number associated with the original frame. Similarly, if you apply a display filter, and then save the filtered frames, the frame numbers in the capture file begin with 1. If, however, you use the Print to File option in the Print dialog box, the original frame numbers associated with the frames are preserved.

Buffer size settings

The default buffer size is 1 megabyte (MB). To increase the buffer size so that you do not lose information, follow these steps:

1.	Click Capture, and then click Buffer Settings.
2.	Increase the value for the Buffer Size (MB) setting, and then click OK.

Tracing in a WAN Environment

Sometimes, you may be asked to make a capture of network traffic between two specific computers that are separated by one or more routers. In these cases, the support professional may want to analyze all network traffic between the first computer and its nearest router, and all network traffic between the second computer and its nearest router. Most of the time, this is done to check whether or not network packets are being lost or corrupted somewhere between the routers. To make these traces consistent and to be able to read these traces simultaneously, the system clocks must be synchronized between the two computers prior to making the trace. Use the following steps to synchronize time between two computers:

1.	Choose the computer against which to synchronize the time.
2.	From the other computer, type the command net time \\ComputerName /set /yes where ComputerName is the name of the computer from step 1.
3.	Verify the computers have the same time by typing TIME at each one.
4.	Proceed with the trace.

Finding Media Access Control Addresses

If the computer to be monitored is running:

•	An MS-DOS-based network client, run MSD at that computer.
•	Windows for Workgroups 3.11 (running TCP/IP), type IPCONFIG /ALL from the command line.
•	Windows 95, run WINIPCFG from the command line at the local workstation.
•	MacOS, Open Appletalk Control Panel. Select User Mode from Edit Menu, change mode to Advanced. Appletalk Control Panel now reveals Info button. Click this button to obtain the MAC address
•	Windows NT, at the local console, use one of these options: • NET CONFIG SERVER from the command line • IPCONFIG /ALL from the command line • IPXROUTE config from the command line • arp -a from the command line • Getmac.exe from the Windows NT Resource Kit • WinMSD
•	Windows NT, remotely, run Getmac.exe from the Windows NT Resource Kit