SQL Server Management Studio connection issue

I am using SQL Server 2008 Enterprise on Windows Server 2008 Enterprise. The server runs SQL Server 2008 is machine M1. I have tried using SQL Server Management Studio on machine M2 could be connected to M1. But I can not use another machine M3 to connect to M1. Any ideas what is wrong? I am not sure whether it is the issue at M1 side or M3 side? Any check list?

Error message is, provider: named pipeline provider program, error 40 -- can n开发者_如何学Pythonot open connection to SQL Server (Microsoft SQL Server: error 53).

thanks in advance, George

On M1 you need to enable TCP/IP in SQL Server Configuration Manager, under "Protocols for MSSQLSERVER". In addition to this you need to open at least port 1433 in the Windows Firewall