NAT hairpinning
also: NAT loopback, NAT reflection, hairpin NAT
NAT hairpinning is the ability of a device behind a NAT to connect to another device on the same private network using that device's external (public) IP address and port, with the NAT router reflexively translating the traffic back through itself.
NAT hairpinning (also called NAT loopback or NAT reflection) occurs when traffic originates and terminates on the same NAT device. Normally, a device inside a private network must use internal IP addresses to communicate with other internal devices. Hairpinning allows it to instead use the external public address, with the NAT router recognizing the outbound connection as destined for itself and redirecting it back into the private network.
A practical example: your home server runs a web service on 192.168.1.100:8080, and your router's public IP is 203.0.113.50. You've port-forwarded port 8080 from the public side to the internal server. Without hairpinning, a device on your home network cannot reach 203.0.113.50:8080—it must use 192.168.1.100:8080 directly. With hairpinning enabled, the router detects the connection to its own public address and intelligently reflects it back to the internal server.
Not all routers support hairpinning by default; enabling it in the router's configuration may be necessary for seamless internal access to externally-facing services like game servers or self-hosted applications.