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To change the MTU for an adapter :

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Class \NetTrans \{Adapter-Id}]
MaxMTU = "576"

The {Adapter-Id} can have the value 0000, 0001, 000n. If several {Adapter-Id}, then find the correct one using the following ways:

• Looking at the value "DriverDesc" for each {Adapter-Id}, and see if it matches the wanted device (Ex. if modem then "Dialup Adapter")
• Change the TCP/IP setting for the adapter in network properties, example the DNS address. Then find the {Adapter-Id} which has that certain DNS-address

More Info MS KB Q250663

Note with DUN 1.3+ (Windows 98 and Windows Me has it installed by default) it is possible to set the MTU for a dialup adapter, without going into the registry. Control Panel -> Network -> Dial-Up Adapter -> Advanced tab -> IP Packet Size.

• Automatic = 0 (576 bytes when below 128 kbps else 1500 bytes)
• Large = 1500 bytes
• Medium = 1000 bytes
• Small = 576 bytes
• Custom (Cannot set MTU below 576 bytes, without using registry editor)

It is possible to change the values for the predefined "IP Packet Size" :

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Class \Net \{Dialup-Adapter-Id} \Ndi \params \IPMTU \enum]
"1492"="Automatic"

More Info MS KB Q183437

Related Finding the optimal TCPIP max transfer unit (MTU) size