Port 1701 — UDP
L2TP tunneling, commonly combined with IPsec.
What is port 1701 used for?
L2TP VPN tunneling.
Transport protocol
UDP
Security precautions
L2TP alone does not provide encryption. Protect the tunnel with IPsec.
Should we open this port in the firewall?
Allow this port only when a required service uses it. Apply least privilege: restrict authorized sources, destinations and interfaces, and log connections where appropriate.
How do I check this port?
Test-NetConnection -Port tests TCP, not UDP. Reproduce an authorized L2TP/IPsec connection and inspect VPN logs and the UDP traffic on both peers.
Capture UDP 1701 during a real application exchange and verify the request and response. UDP has no TCP-style handshake; a missing reply to a generic probe does not prove that the port is closed.
Port-to-service mappings are common conventions. Applications can use a different configured port; confirm the actual service and transport.