Which protocol usually requires open ports to establish a VPN connection?

OpenVPN protocol usually requires open ports to establish a VPN connection. By default, OpenVPN uses UDP port 1194, but it can also operate over TCP on other ports. For successful connection, the corresponding port must be open on the server and not blocked by firewalls.

Other VPN protocols, such as IPSec, may use multiple ports and protocols (UDP 500, UDP 4500, etc.), but OpenVPN is most known for its dependence on an open port for operation.