You'll need to use SQL Server configuration Manager to set SQL server's IP bindings. I've had to do something similar to meet a company policy. This is what I ended up doing.

Under SQL Serer 2005 Network Configuration will be a listing for each instance in the right will be a listing of the Protocols.

Under each instance open the properties of TCP/IP

1)Set "Listen All" = No.

2)Set the desired IP and port

3)Disable dynamic ports by removing '0' from TCP Dynamic ports for all IP's except for 127.0.0.1 and "IPAll"

4)Shutdown all instances of SQL

5)Start up all instances of SQL

Any errors can be troubleshot using the errorlogs for each instance (.MSSQL\LOG\ERRORLOG)