How to solve it when there are multiple SQL Server instances, but can only connect to one?

Question

Using SQL Server 2008 R2

I have 2 SQL Server instances on my PC, SQL1 and SQL2.

I have Windows XP installed in a VirtualBox. In XP, I have an application which can connect to SQL Servers.

When I try to connect to SQL1, everything is fine, but when I try to do that with SQL2 it errors out with

Could not open a connection to SQL Server

So I started checking the settings. I can connect to SQL2 using SQL Server Management Studio (on a local PC), so I checked that 'Allow remote connections to this server' was enabled. Also, in SQL Server configuration tool I enabled all protocols for this instance, just as it is for the SQL1. SQL Server browser is working.

Configuration of both instances seem to be identical. There is only one difference between SQL1 and SQL2 that I have noticed.

• SQL1 version is 10.50.4000
• SQL2 version is 10.50.1600

What can cause one of the instances to be unreachable through the network while the other one can be, and both settings are the same?

Answer 1

It could be firewall settings or network settings. For an instance to be reachable, configure at least the following.

• TCP protocol enabled (SQL Server Configuration Manager)
• Fixed TCP port (SQL Server Configuration Manager)
• Enable SQL Server Browser Service
• Open firewall for SQL Browser service (UDP port 1434) if you have a named instance.
• Open TCP ports for SQL Server Instances.

See Configure the Windows Firewall to Allow SQL Server Access