1701
L2TP VPN tunneling protocol, typically combined with IPsec
Layer 2 Tunneling Protocol
What is port 1701?
UDP port 1701 is used by L2TP (Layer 2 Tunneling Protocol). L2TP itself provides no encryption and must always be paired with IPsec (L2TP/IPsec) for a secure VPN. It is built into Windows, macOS, iOS, and Android as a native VPN option. While secure when properly configured with IPsec, it has reportedly been cracked by the NSA when using pre-shared keys with weak configurations; WireGuard or IKEv2 are preferred for new deployments.
Quick checks
$
nc -zv example.com 1701
$
nmap -p 1701 example.com