The default Maximum Transmission Unit (MTU) size for PPPoE (DSL) connections in Windows XP is typically set to 1,480 or 1,490 bytes. However if find you can’t browse some Web sites or send e-mail messages with attachments, your DSL connection may work better with a slightly lower MTU size setting (somewhere between 1,400 to 1,490). You can refer to another tip on this site for information on how to test out what MTU size is best for your Internet connection.

To edit the MTU size in Windows XP, you must first add the following keys to the Windows Registry:

Hive: HKEY_LOCAL_MACHINE
Key: System\CurrentControlSet\Services\Ndiswan\Parameters\Protocols\

Hive: HKEY_LOCAL_MACHINE
Key: System\CurrentControlSet\Services\Ndiswan\Parameters\Protocols\0

Then add the following entries to the 0 key:

Name: ProtocolType
Type: REG_DWORD
Value: 0x00000800

Name: PPPProtocolType
Type: REG_DWORD
Value: 0x00000021

Name: ProtocolMTU
Type: REG_DWORD
Value: desired MTU size

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Eric Geier is an author of many wireless networking and computing books including Home Networking All-in-One Desk Reference For Dummies (Wiley 2008) and 100 Things You Need to Know about Upgrading to Windows Vista (Que 2007).