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Can Only Go To Some Websites. Could Be MTU Needs Changing

Client that just bought a new Windows 7 PC having issues getting POP3 email and going to certain websites. They are using a really old PC as their router. I had to lower their MTU value on the PC in order to get everything working.

Normally you don’t have to change the MTU but dealing with older routers or certain ISP’s this will help. Most routers will auto adjust your need.

First thing I had to do is determine what value works. The easiest way to do this is run the following from a dos prompt:

ping www.google.com -f -l XXXX       (thats -F -L but need to use lowercase and XXXX is the MTU value to test).

MTU start as high as 1500 (this is the normal default setting for most routes/PC’s etc). If 1500 does not work (which probably won’t even work with regular setup) you will get a message:

Packet needs to be fragmented but DF set

start to lower the MTU to say 1492. Normally I have to go down to 1460 and then I will get the normal ping response.


Once you know your value then you need to first check the current MTU value and which interface you are using. Run the following:


netsh interface ipv4 show subinterface


netsh interface ip show interface

by default the MTU will be 1500 and the interface is Local Area Connection

Now to change the MTU permanently on boot you will need to Run As Administrator the following command in Windows 7:

netsh interface ipv4 set subinterface “Local Area Connection” mtu=XXXX store=persistent

(Where XXXX is the new MTU value Example 1460)

If you just want to test it then remove the store=persistent. But after reboot it will go back to its default.

If you are on Windows XP then download a utility called DrTCP. It will do this for you in GUI.




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