- 1). Open the Command Prompt, also known as the "DOS prompt," in Windows.
- 2). Type "ping www.expedient.net --f --l 1492" and press the "Enter Key." The functions "-f --l" are typed as "minus f space minus l." This command sends a packet of data to expedient.net and determines if the packet is fragmented or lost.
- 3). Examine the returned values. The ping test will respond with either a "Packet needs to be fragmented," or "Reply from...." If the packet needs to be fragmented, the MTU setting of 1492 is too low.
- 4). Repeat the ping test with a increasingly lower or higher packet sizes by replacing 1492 with a different value. The optimal setting is found when the packet size is the lowest possible value without fragmenting. Change the numbers by 10 or more to reduce the number of attempts made to initially bracket the high and low values around the optimal MTU setting.
- 5). Add 28 to the final number of the optimal packet size. The added 28 accounts for the IP/ICMP headers always sent with packets. The new number is the optimal MTU setting for the connection. For example, if the lowest value of the ping test successfully returns a "Reply from..." response of 1462, adding 28 reveals that 1490 is the optimal MTU setting.
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