- Error number
- 26
- Severity
- 20 (fatal: the connection ends; may indicate a serious problem)
- State
- 0
- Product
- Microsoft SQL Server Database Engine
- Versions
- Any SQL Server version reached through SQL Server client drivers (ODBC, OLE DB, SqlClient) using a named instance
- Category
- Connectivity
- Last reviewed
- 23 October 2026
On this page
What it means
Error 26 is a client-side error: the driver could not find the named instance you asked for, usually because the SQL Server Browser service did not answer on UDP 1434 or the instance name is wrong.
This page covers the client error 26 raised by the SQL Server Network Interfaces layer of the driver. It is not a SQL Server engine message: sys.messages has no message 26, and the engine never sees the attempt. A named instance (SERVER\INSTANCE) usually listens on a dynamic port, so before connecting the driver asks the SQL Server Browser service on the server, over UDP port 1434, which port the instance uses. When no answer comes back, or the answer does not list that instance, the driver gives up with error 26.
Read the provider part of the message: SQL Network Interfaces, error: 26 means the lookup of the instance failed, before any TCP connection to SQL Server. With ODBC Driver 18 the line reads Error Locating Server/Instance Specified [xFFFFFFFF] and is followed by Login timeout expired. Compare error 53 and error 40 (the server or pipe cannot be reached at all) and error 10061 (the port refused the connection).
Common causes
- The instance name is misspelled, or the instance is a default instance and must be addressed without
\INSTANCE. - The SQL Server Browser service is stopped or disabled on the server (it is needed only for named instances).
- A firewall or router drops UDP 1434 between the client and the server.
- The instance is hidden from SQL Server Browser, or the SQL Server service itself is not running.
- The client resolves the server name to the wrong machine (stale DNS, HOSTS file entry or client alias).
Diagnose it
Read-only These queries only read metadata and dynamic management views. They do not change data, permissions or configuration, so they are safe to run first.
Get the real instance name and port (on the server)
Run locally on the server, where a shared memory connection does not need the Browser.
SELECT SERVERPROPERTY('MachineName') AS machine_name,
SERVERPROPERTY('InstanceName') AS instance_name, -- NULL = default instance
SERVERPROPERTY('ServerName') AS server_name;instance_name NULL means a default instance: connect with the machine name alone. Otherwise use exactly machine_name\instance_name.
Which TCP port does the instance listen on?
SELECT ip_address, port, type_desc, state_desc
FROM sys.dm_tcp_listener_states
WHERE type_desc = 'TSQL';The port to put in the connection string (SERVER\INSTANCE,port or tcp:SERVER,port) to bypass the Browser. Rows only on 127.0.0.1 or ::1 port 1434 are the dedicated admin connection, which means TCP/IP for clients is not enabled.
Check the Browser service and UDP 1434
On the server, in PowerShell: Get-Service SQLBrowser should show Running. From the client, Microsoft suggests PortQry (or PortQryUI) against UDP 1434; a FILTERED result means the port is blocked on the way.
How to fix it
Each fix is labelled with what it changes. Try read-only checks and the least invasive fix first, and test changes outside production.
Correct the server and instance name
Client or network sideUse the exact name from SERVERPROPERTY('ServerName'). For a default instance drop the \INSTANCE part.
Connect with an explicit port instead of the instance name lookup
Client or network sideAdding the port skips the Browser lookup entirely. This is also the way to confirm the diagnosis: if the port works and the name does not, the Browser or UDP 1434 is the problem.
sqlcmd -S tcp:SQLPROD01,50123 -E
-- or in a connection string: Server=SQLPROD01\SALES,50123Start SQL Server Browser and allow UDP 1434
Changes configurationOn the server, set SQL Server Browser to start automatically and allow inbound UDP 1434 in the firewall, so clients can resolve named instances.
Set-Service -Name SQLBrowser -StartupType Automatic
Start-Service -Name SQLBrowser
New-NetFirewallRule -DisplayName 'SQL Server Browser' -Direction Inbound -Protocol UDP -LocalPort 1434 -Action AllowGive the instance a static port
Changes configurationWhere UDP 1434 cannot be opened, configure a static TCP port in SQL Server Configuration Manager, open that port in the firewall and put it in connection strings. Changing the port needs a service restart.
Caution: Restarting the SQL Server service disconnects every session; schedule it.
Examples
A named instance that does not exist on the machine
Reproduced with sqlcmd 17.0.1000.7 and ODBC Driver 18 on 2026-10-23 against the local machine, where no instance called CWSNOSUCHN5 exists. The driver waits for the login timeout (5 seconds here) before reporting.
sqlcmd -S .\CWSNOSUCHN5 -E -C -l 5Sqlcmd: Error: Microsoft ODBC Driver 18 for SQL Server : SQL Server Network Interfaces: Error Locating Server/Instance Specified [xFFFFFFFF]. . Sqlcmd: Error: Microsoft ODBC Driver 18 for SQL Server : Login timeout expired. Sqlcmd: Error: Microsoft ODBC Driver 18 for SQL Server : A network-related or instance-specific error has occurred while establishing a connection to .\CWSNOSUCHN5. Server is not found or not accessible. Check if instance name is correct and if SQL Server is configured to allow remote connections. For more information see SQL Server Books Online..
How to prevent it
Document every named instance with its static port, and use SERVER\INSTANCE,port or tcp:SERVER,port in application connection strings so they do not depend on SQL Server Browser and UDP 1434.
Include the Browser service and the firewall rules in server provisioning scripts, and test new connection strings with sqlcmd from the application host before go-live.
Common questions
Is error 26 a SQL Server error?
No. It comes from the client driver (SQL Server Network Interfaces) while it looks up the named instance. SQL Server never receives the connection, so there is nothing in the SQL Server error log.
Why does the connection work with a port number but not with the instance name?
The instance name is resolved to a port through SQL Server Browser on UDP 1434. If the port works, the instance is fine and the Browser service or UDP 1434 is the problem.
Do I need SQL Server Browser for a default instance?
No. A default instance is reached on TCP 1433 (or the port you give) without the Browser. Adding a wrong \INSTANCE suffix to a default instance name causes error 26.
Learn more on CodeWithSQL
Official documentation
- Fix network-related or instance-specific errors in SQL Server (Microsoft Learn)
- SQL Server Browser service
- Configure a server to listen on a specific TCP port
Last technically reviewed 23 October 2026; examples and diagnostics run on SQL Server 2025 (17.0.1145.4), ODBC Driver 18 for SQL Server. Message text checked against the engine's sys.messages catalogue.