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If you are able to access almost all web sites but have any or all of the following symptoms your MTU (Maximum Transfer Unit) may be incorrect.
Can not access certain web sites or see particular frames within a website.
Can not access web sites or sections of web sites that require a username and password (i.e. banking, stock quote, online ordering, etc).
Can not access or download from certain FTP sites.
Can not access certain servers or information while using a VPN client.
Can not access web based email sites or send/receive emails.
Can not access POP based email or send/receive emails.
Can not send/receive file transfers.

To find the correct MTU for your configuration you must run a simple DOS Ping test. Please reference the following steps:

The command for this ping test is ping www.dslreports.com -f -l xxxx.
•There is a single space between each command.
•"-l" is a lower case letter L, not the number one.
•The last four numbers are the test packet size.

Open a DOS prompt screen by clicking on Start>Programs>MSDOS-PROMPT. You can also use the Run Command by clicking on Start>Run then type in "command" for Windows 95/98/ME or "cmd" for Windows 2000/XP.
At the DOS Prompt type in ping www.dslreports.com -f -l 1472 and hit Enter. Notice that the packet needs to be fragmented. (Figure 1)

Drop the test packet size down (10 or 12 bytes) and test again. Notice that the packet still needs to be fragmented. (Figure 2)

Drop the test packet size down more and test again until your reach a packet size that does not fragment. (Figure 3)

Once you have a test packet that is not fragmented increase your packet size in small increments and retest until you find the largest possible packet that doesn't fragment.

Take the maximum packet size from the ping test and add 28. You add 28 bytes because 20 bytes are reserved for the IP header and 8 bytes must be allocated for the ICMP Echo Request header.

An example:
1440 Max packet size from Ping Test
+ 28 IP and ICMP headers
1468 Your optimum MTU Setting

You can download and use DrTCP or any similar registry editing application to change your MTU. Remember, if you have more than one network adapter on a PC you must change the MTU on the correct network adapter associated with your broadband connection. Additionally, if you have a network with more than one PC, all computers must have the same MTU setting. Note: You must reboot your machine in order for the new MTU settings to take place. An example of changing the MTU using DrTCP is shown below. (Figure 4)

Andy Houtz

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by Andy Houtz See Profile edited by KeysCapt See Profile
last modified: 2003-02-18 19:17:41

Also read About DSL for lots more information