A bastion host is a server that is fully exposed to the public Internet, but is extremely hardened to prevent attacks and is usually dedicated for a specific application or usage; it is not something that will serve multiple purposes. This singular focus allows for much more stringent security hardening and monitoring.

Explanation
Hex GBL is a reference to a computer part in Terry Pratchett's fictional Discworld universe. The rest are not.

Explanation
Virtual private networks (VPNs) are used to provide administrative personnel with secure communication channels through security systems and into trust zones. They allow staff who perform system administration tasks to have access to ports and systems that are not allowed from the public Internet. IPSec is an encryption protocol for point-to-point communications at the network level, and may be used within a trust zone but not to give access into a trust zone. TLS enables encryption of communications between systems and services and would likely be used to secure the VPN communications, but it does not represent the overall concept being asked for in the question. SSH allows for secure shell access to systems, but not for general access into trust zones.