Setting the optimal MTU inside of your router

How you find the optimal MTU settings is truly a matter of opinion.  Here is what we do…remember setting firewalls, routers and your computer will truly tweak out your connection.  Start on the outside and change your router or firewall first and then each computer (Use TCPOptimzer for computers).  Don’t forget access points and other devices.

  • Find an ISP or a website you can ping.
  • Ping the website from the command prompt (Windows) using the following command
  • ping -f -l 1472 http://www.yourwebsiteaddress.com   (the -l is an “L” that is a lowercase and there is a space between each command and switch)
  • Hit the enter key
  • If you receive the message of fragmentation, reduce by 10 or 20 until you get responses.  Increase the number by 1 until you are 1 less away from getting a fragmentation message (midpoint – between high and low)
  • Now add 28 to this number.  (Why? This is because you specified the ping packet size – and did not include the header which is 28bytes)
  • This will give you your MaxMTU size.  (Enter this into your devices and into your OS)

Other Operating Systems

Linux users

ping -s 1472 www.yourwebsiteaddress.com

Apple users:

ping -D -s 1472 www.yourwebsiteaddress.com

Linux and Apple commands are case sensitive.

Here’s a quick example where I ran all the way down to 1400.  Therefore I would add 28bytes and the MaxMTU would be 1428.

C:\Users\x2370>ping -f -l 1450 http://www.somewhereoverthere  (use your ISP here)

Pinging http://www.somewhereoverthere [ IP Address] with 1450 bytes of data:
Reply from 192.168.1.1: Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for  IP Address:
Packets: Sent = 4, Received = 1, Lost = 3 (75% loss),

C:\Users\x2370>ping -f -l 1400 www.somewhereoverthere  (use your ISP here)

Pinging http://www.somewhereoverthere IP Address] with 1400 bytes of data:
Reply from IP Address: bytes=1400 time=161ms TTL=110
Reply from  IP Address: bytes=1400 time=80ms TTL=110
Reply from  IP Address: bytes=1400 time=60ms TTL=110
Reply from  IP Address: bytes=1400 time=124ms TTL=110

Ping statistics for  IP Address:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 60ms, Maximum = 161ms, Average = 106ms

Always test your internet before and after.  Use a true speed test such as Testmy.net

Testmy.net has been around since 1996 and uses real world speed test.  Testmy.net also is NOT owned by an ISP so there isn’t a chance of inflated scores or speeds.

Give it a try and use the Download and Upload link from the menu.

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About TCAT Shelbyville IT Department

The Tennessee College of Applied Technology - is one of 46 institutions in the Tennessee Board of Regents System, the seventh largest system of higher education in the nation. This system comprises six universities, fourteen community colleges, and twenty-six Applied Technology Colleges.
This entry was posted in Apple, Computer hardware, Computers, Home Networking, Information Technology, Internet, network, Networking, Technology, Ubuntu, Windows, Wireless and tagged , , , , , , , . Bookmark the permalink.

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