Topics from the Microsoft SQL Server Protocols team - Netlibs, TDS, SQL Browser, etc.
In order to better assist troubleshooting connectivity related issues, the more specific you describe the problems, the faster we can help you out. Please take a few minutes to answer the following questions and post them along with your specific repro steps.
Check out this blog site first, it may already have your questions answered. Otherwise, you can post your questions on
 Client side:
In cmd.exe console, type “ping -a <server_name>”.
In cmd.exe console, type “telnet <server name> port, where port can be 135, 445 or sql_server_tcp_port. If your cmd.exe console turns into a complete black screen with a cursor flushing on top left corner, you are connected. Type ctrl+’[‘ to bring up telnet prompt and type “quit” <enter>.
 Server side:
1. Find SQL Server Errorlog: Default to C:\Program Files\Microsoft SQL Server\MSSQL.#\MSSQL\LOG
 Last but not least:
There is a wealth of information already available to help you answer your questions.
(1) SQL Server 2005 Books Online: http://msdn2.microsoft.com
(2) Microsoft Support Knowledge Base: http://support.microsoft.com
(3) SQL Protocol Team Blog: http://blogs.msdn.com/sql_protocols
(4) MSN Search: http://search.msn.com/ or use your favorite search engine.