Internet Connectivity Problem: Setting the Right MTU size


Many of us have experienced the problem with a new router which blocks certain sites unexpectedly, and many a times we blame the retail guys for the same. But little do we know that  this mainly happens due to something called as Maximum Transmission Unit (MTU). Maximum Transmission Unit (MTU) is an advanced configuration on a router which allows it to determine the largest data size permitted on your connection. If the size of MTU for your connection is set too high, (which generally happens with new routers) then your computer will experience dropping internet connection and block certain sites. So it is necessary to determine the correct MTU size and set the same for your connection.

Determining the optimum MTU size.

To determine the correct MTU size for your router you will have to perform what is called the ping test on another computer or a web address using the command prompt on your system. ( Ping is a command used to determine the connectivity between your computer and another address )


For Windows users, the following command can be used for ping test

ping [url / IP address]–f –l xxxx

e.g: ping –f –l 1472

Mac users need to use the following command for the ping test

ping [url / IP address]–d –s xxxx
e.g: ping –d –s 1472

URL is the web address with which you are trying to determine the connectivity of your computer.

 -f is a command parameter which makes sure that the packet sent or received isn’t fragmented.

 -l is a command known as a packet size switch.  This helps you to determine the best MTU size for your network.

xxxx is the packet size (value) with which you shall test the connectivity. Starting with 1472, this value can be decreased by 10 units each time until you don’t get rid of the statement saying “Packet needs to be fragmented but DF set.”

With all this info, now we go ahead to determine the correct MTU size for a connection.

If you are a windows user, you need to open the command prompt on your system.
Mac users need to open the terminal.
( The images below indicate the command and its working on a windows environment. Make sure that you use the correct commands for your operating system.)

1)      Once you open the command prompt you need to enter the ping command, for the url we shall be using

Note: In the image below the initial MTU size is set at 1472.

mtu 1

2)  If you do not get a response, decrease the packet size by 10 and enter “ping –f –l 1462”

mtu 2

3)  Continue to decrease the packet size by 10 units until you get a reply.

mtu 3


Once you get a reply, take the value (1452 in this case) and add 28 to it to get the optimum MTU size for your connection. In this case 1452+28= 1480 is the optimum MTU setting.


Setting the MTU size for a router

Now in order to set the determined MTU size for the router, you will have to log in to the router’s web-based setup page using your internet browser (Usually or and enter the user credentials.



Once there, go to the Internet Settings tab and locate the MTU settings box.

mtu 5


Now just change the MTU value with the Determined value.

Note: If this still doesn’t help you, Contact your service provider for the correct MTU size.


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