Definition
A network port is a 16-bit numerical identifier (0 to 65535) that distinguishes one application's connection from another's on the same device. When a packet arrives at a host, the operating system uses the destination port to decide which application gets it: port 80 for unencrypted web traffic, 443 for HTTPS, 25 for SMTP email, 22 for SSH. Combined with an IP address, a port forms a 'socket' that uniquely identifies one endpoint of one connection.
Ports and VPNs
VPN protocols use specific ports by convention. OpenVPN: UDP 1194 by default, often configurable. WireGuard: UDP 51820 by default. IKEv2/IPSec: UDP 500 and 4500. PPTP: TCP 1723 (legacy). Restrictive networks frequently block these well-known VPN ports as a first line of defence against VPN use. The standard countermeasure is to run the VPN on TCP 443 — the HTTPS port — which is virtually never blocked because doing so would break the web.
Port Forwarding
Port forwarding is the practice of mapping an inbound port on a public IP to a specific port on a private device behind NAT. Some VPN providers offer port forwarding as a feature so that users behind the VPN can run servers reachable from the open internet — important for BitTorrent seeding, hosting game servers, or remote desktop. Without port forwarding, the VPN's NAT blocks all unsolicited inbound connections.