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Most people do not need to install the Microsoft Loopback Adapter to connect to SQL Server on the same computer. The key distinction is that the Windows software adapter—now called the Microsoft KM-TEST Loopback Adapter—is not the same as the built-in loopback address 127.0.0.1. SQL Server can also accept local connections through shared memory, without using a network adapter.
What is the Microsoft Loopback Adapter?
The Microsoft Loopback Adapter is a virtual Windows network adapter that can be added to a computer. Microsoft renamed it the Microsoft KM-TEST Loopback Adapter in Windows 8 and Windows Server 2012. Microsoft’s installation instructions describe adding it with the Add Hardware Wizard.
Its name is easy to confuse with 127.0.0.1, the loopback IP address. They are distinct: 127.0.0.1 is an address used by programs on the same computer to reach that computer, not the name of an extra adapter you must install. Microsoft explains this local-only behavior in its SQL Server TCP/IP troubleshooting guidance.
How SQL Server connects locally
SQL Server can run without a network connection. For a client on the same Windows computer, the available connection protocols include shared memory, named pipes, and TCP/IP. Shared memory is the fastest option in Microsoft’s overview of SQL Server network protocols because it does not use the network interface.
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| Connection method | Where it works | What it uses |
|---|---|---|
| Shared memory | Same computer only | Local communication without the network interface |
TCP/IP to localhost or 127.0.0.1 |
Same computer | The network stack; SQL Server must be listening for TCP/IP |
| Remote TCP/IP | Across a network | A listening SQL Server endpoint, the appropriate port, and firewall access |
For local TCP/IP connections, Microsoft’s connection troubleshooting guide gives examples using localhost and 127.0.0.1. A named instance, custom port, or SQL Server Browser configuration can affect the connection string.
When might SQL Server use loopback networking?
If an application or test specifically needs TCP/IP, it can connect to SQL Server through a local endpoint such as localhost or 127.0.0.1, as long as TCP/IP is enabled and SQL Server is listening. This uses the loopback address; it does not, by itself, require adding the KM-TEST adapter.
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- Connects a USB 3.0 device (computer/laptop) to a router, modem, or network switch to deliver Gigabit Ethernet to your network connection. Does not support Smart TV or gaming consoles (e.g.Nintendo Switch).
- Supported features include Wake-on-LAN function, Green Ethernet & IEEE 802.3az-2010 (Energy Efficient Ethernet)
- Supports IPv4/IPv6 pack Checksum Offload Engine (COE) to reduce Cental Processing Unit (CPU) loading
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There are specialized configurations where the loopback IP is useful. Microsoft’s multi-homed SQL Server guidance describes using a computer’s adapter address and 127.0.0.1 to simulate two network adapters. That is a particular network configuration, not a general prerequisite for SQL Server.
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What to check if a SQL Server connection fails
- Determine whether the client is local or remote. A local client can use shared memory or a local network endpoint. A client on another computer needs a network protocol and a reachable server endpoint.
- Check the instance’s protocols. Use SQL Server Configuration Manager to inspect and configure protocols. Shared memory is enabled for local connections, while TCP/IP can be disabled by default in some new installations and editions. Defaults vary, so check the instance rather than assuming TCP/IP is active.
- For a local TCP/IP connection, use the right endpoint. Try the appropriate local name or address, and account for a named instance or custom port in the connection string.
- For a remote connection, verify the listener and firewall. Confirm SQL Server is listening on the expected port and that the firewall permits the connection. Microsoft’s Windows Firewall guidance explains how firewall configuration can block SQL Server access.
- Add the KM-TEST adapter only for a specific networking requirement. It is an optional Windows virtual adapter, not a standard requirement for routine local SQL Server connections.
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