At the very end of MSKB 158474 is a reference to what does need to be done to mess around with the MTU.

Seems you need to add a registry entry MaxMTU to the following registry key where n is the particular TCP/IP-to-network adapter binding. On my system I have only one NIC and it is 0001

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\netTrans\000n

The default MaxMTU value for the datagram excluding SNAP and routing headers is indeed 1500 on an Ethernet network unless the media driver sets another value, in which case the minimum of the two is used.

So, the real question seems to be what value is the media driver setting anyway and which could already be a value less than 1500.

But wait there is more - as they say in the TV ads:

The article states:

"In an Ethernet network, MaxMTU will default to 1500."

So far so good. Buried somewhere in the internals of Windows is a value of 1500 which is used to set the max datagram size, unless it is replaced by the value in a registry key named MaxMTU.

"The actual value used will be the minimum of the value specified with this parameter and the size reported by the media driver."

Still OK even though no light is shed on the media driver or how to find the size its reporting.

"The default is the size reported by the media driver."

This flat out contradicts the first statement.

Is it possible then that all we achieve by using Ping to find the maximum undefragmented datagram value is in fact to discover the value reported by the media driver and which is being used anyway?