How to check MTU size under Vista ?

To check your Vista MTU settings, open command prompt as administrator, and type the following command:

netsh interface ipv4 show subinterface

You will be presented with a list of active network interfaces similar to the following:

MTU MediaSenseState Bytes In Bytes Out Interface
------ --------------- --------- --------- -------------
4294967295 1 0 200260 Loopback Pseudo-Interface 1
1500 1 204234 1440183 Wireless Network Connection
1500 5 0 0 Local Area Connection

Note the corresponding MTU value in the first column.

See also: How to set the MTU size under Vista ?