Troubleshooting Protocol Configuration Problems
Many, but not all, of the problems you encounter with remote connectivity can be addressed with the measures listed previously. However, you might encounter a problem when you have confirmed that the network user is using the correct username and password combination, that no changes have been made to the user's account information, that all physical connections are in place, and that the user still cannot establish a remote connection.
The next most likely cause of a client connectivity problem is protocol configuration. Protocol configuration issues are usually on the client side of the network. On a TCP/IP network, each client computer must have a unique address in order to participate on the network. Failure to obtain addressing information automatically could indicate a problem with a DHCP server. You should check the DHCP server to make sure that it is functioning and that addresses are available for assignment.
Beyond basic protocol issues such as addressing, remote connectivity troubleshooting also brings with it the additional considerations of authentication protocols. There is one basic rule that applies to all such issues. If a client in a remote connectivity solution is configured to use one type of authentication protocol, and the server to which he is connecting does not support that protocol, the connection will be refused.