SQL Server Error 26 relates only to SQL Server Named instances. Not default instances
The main reason SQL Server Error 26 error message appears is that the client can’t receive SQL Server Resolution Protocol(SSRP) response UDP packet from the SQL Browser Service
1)Check the server name is correct?Check spelling.
2)Check the SQL Server instance name, is there such an instance on the target server?
3)Can you ping the server?Is DNS resolvable?Sometimes ping isn’t possible as the administrator disabled the ping service.
4)Is SQL Browser Service turned on?
5)Is the frewall enabled?If so, place sqlbrowser.exe or port 1434 into the exception list.
6)Is you server part of a Cluster?
If after these steps the connection is still failing try:
Add a comment, if you have a problem – and I’ll assist in troubleshooting. Or another SQL Server DBA will help
This will give you some extra information to troubleshoot the SQL Server Error 26 message
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