I got a call from a client that uses an MS Access database as a front end to a SQL Server 2008 back end. Suddenly they said that they could not access the database. I knew that recently their IT department had added a domain controller and joined the server to the domain. It turned out that the Group policy had turned on the firewall and ports TCP 1433 and UDP 1434 were now being blocked. Port UDP 1434 needs to be open if you connect to the server by using only the name of the instance so in this case exceptions were added to the firewall rules to open both ports.
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sql server ports