PPPoE is a method for establishing PPP connections through Ethernet network adapters. See RFC2516 : Transmitting PPP Over Ethernet PPPoE. Many DSL providers use PPPoE to maintain the familiar "dial-up" experience when users connect to the Internet through a DSL modem (and to control the number of their IP addresses used by their DSL users).

PPPoE requires a MTU ( Maximum Transmission Unit ) setting on all client computers of 1492 or less. This includes any computers you have accessing the Internet via an ICS ( Internet Connection Sharing ) connection. The symptom when the MTU is too large is an inability to browse some Web sites or send e-mail messages that contain attachments. To check your MTU setting:

• Determine the default gateway of the ICS server using ipconfig
• From a client, type ping -f -l MTU size IP address of default gateway from first step
• Start with an MTU size of 1454. If this fails with an error message indicating that it must be fragmented, decrease the MTU size and try again. Repeat this until the ping command succeeds.