What is "hairpinning"

In network computing, hairpinning (or NAT loopback) describes a communication between two hosts behind the same NAT device using their mapped endpoint. Because not all NAT devices support this communication configuration, applications must be aware of it.

Hairpinning is where a machine on the LAN is able to access another machine on the LAN via the external IP address of the LAN/router (with port forwarding set up on the router to direct requests to the appropriate machine on the LAN).