Ok, I have 6 servers for a customer platform. When I use the IP to connect using mstsc (microsoft terminal server client), it asks me user and password and after that says that credentials are not valid.
Attemping the connection using servername.domain, works fine. The nslookup translation works fine and the IP is correct. I'm not using IPv6. I experience the problem using the mstsc even between the servers.
Using windows xp attemping the connection using the IP works, but with w2008 server r2 or windows 7, it doesn't.
The problem is that the DNS is private and for the servers only. For external users the access is restricted using a dedicated IP for the client to connect.