Well , I'd Say that you keep getting disconnected , because the telnet service on the computer your telnetting into doesn't have a telnet server up and listening . The thing about telnet is that when you don't specify any ports , it automatically telnets to port 23 (telnet port). In the future you'll hear alot about SSH (secure shell) , Which is different than telnet , because your user id and password are encrypted across the line , so therefore it is more secure than telnet. But until then , pick up a book on tcp/ip.
Later.