- Run SQL Server Configuration Manager.
- Go to SQL Server Network Configuration > Protocols for SQLEXPRESS.
- Make sure TCP/IP is enabled.
- Right-click on TCP/IP and select Properties.
- Verify that, under IP2, the IP Address is set to the computer's IP address on the local subnet.
- Scroll down to IPAll.
- Make sure that TCP Dynamic Ports is blank. (Mine was set to some 5-digit port number.)
- Make sure that TCP Port is set to 1433.
Note: if you follow these steps, it's not necessary to enable SQL Server Browser, and you only need to allow port 1433, not 1434.