IPConfig Shows Default Subnet Mask and Gateway for RAS Client (138238)

SUMMARY

This article explains the results that appear when you run IPCONFIG.EXE (WINIPCFG.EXE in Windows 95) from the Remote Access (RAS) client computer when you use Windows NT 3.5, 3.51, or Windows 95 as a PPP RAS client and dial into a Windows NT RAS server or any Windows NT-compatible PPP server using the TCP/IP protocol.

MORE INFORMATION

The following is an example of what appears on your screen when you run IPCONFIG.EXE (WINIPCFG.EXE in Windows 95) on your RAS client:

Ethernet adapter NdisWan4:

IP Address. . . . . . . . . : 130.80.0.51
Subnet Mask . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . : 130.80.0.51

The IP address for the NDISWANx (where x is the number of the NDISWANx instance) adapter is the IP address of the RAS client. However, the subnet mask does not match the subnet mask of the PPP server and the default gateway shows the same address as the RAS client IP address.

The subnet mask is not passed by the PPP Server. IPCP is the PPP component that passes TCP/IP address information, and the subnet mask is not defined in the Request For Comment (RFC) as a legal value to pass. RFC 1332 explains IPCP.

In Windows NT 3.5, the subnet mask is displayed as 0.0.0.0. However in Windows NT 3.51, the code has changed to give a subnet mask that is standard to the type of address that is passed by the PPP server. For example, if you receive a Class A IP address, a subnet mask of 255.0.0.0 is displayed.

The RAS client uses the local subnet mask to determine whether to send data over the RAS connection or the local network connection. The default gateway displays the IP address of the RAS connection. This is expected behavior. It is up to the PPP server to forward RAS IP traffic to its own default gateway.