By default, Windows NT attempts to discover the maximum transmission unit (MTU) of all connections beyond your local subnet. Limiting TCP segments to this size reduces fragmentation at the routers along your connection path, increasing your throughput. You may have inadvertantly disabled this feature while tuning your network. Navigate to:
If you have a value name of EnablePMTUDiscovery, a type REG_DWORD that is set to 0, discovery is turned off. The lack of this value means that discovery is turned on. The possible data values are:
0 TCP uses an MTU of 576 bytes for all connections to computers outside the local subnet.
1 TCP attempts to discover the MTU of the path to a remote host. This is the default.