THE MAIN PART OF THE GUIDE WORKS FOR BOTH VISTA AND WINDOWS 7
MTU, or Maximum Transmission Unit, is the max amount of bytes you can accumulate before a packet of data is sent(?). Often, it helps your ping in an MMO decrease a lot due to you being able to send packets much quicker. This combined with disabling the Nagle Algorithm makes for an amazing lag killer.
Honestly, this lag fix is often overlooked by many people. To me, it is VERY easy to do, and can be easily set back to factory default.
Step 1) Run cmd.exe as an administrator.
Step 2) type the command: netsh interface ipv4 set subinterface "Local Area Connection" mtu=xxxx store=persistent
Note: If you use a wireless connection, you would type "Wireless Network Connection" rather than "Local ---"
Note2: The value of the MTU can be anywhere from 48 to 1500(default value). Lower values decrease load time of content heavy pages and games, so a mid-range value is probably best. Also, the value of the MTU has to be a multiple of 4(?). (I set my MTU usually to 168, but 368 is a popular choice also. Content heavy games such as Aion would probably need an MTU of around 568 or so, etc. Whatever floats your boat, haha.)
Step 3) Hit enter...(lol)
If all goes well your MTU should have been changed. This method is only confirmed for Vista OS though, but it wouldn't hurt to try it with a different Windows OS.
Kind criticism is appreciated, but don't flame meeeee D:
Extra IMPORTANT-ish note! Please have a static IP address before trying to change your MTU to anything besides 1500. [GUIDE TO STATIC IP]
Pictures for reference:
If nothing works, I can try a TeamViewer session with you. This is a COMPLETE last resort though.
Windows XP users:
Step 1) Run Regedit
Step 2) Browse to HKEY_LOCAL_MACHINE \SYSTEM\CurrentControlSet\Services\Tcpip\Parameter s\Interfaces\
Step 3) Similar to how you would disable Nagle, locate the folder which has info about your IP address
Step 4) Within that folder, there might be a value named MTU. If there isn't, create a d-word value named MTU. Modify that value and change the number to what please you. You can use Hex:MTU ratio examples that I found such as 96:150, 146:326, and 500:1280
Step 5) Reboot your computer and if a noticeable change is present, you probably changed your MTU! Congrats!
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Edit: Videos taken to show the difference lowering the MTU makes(in Mabinogi).