An internet router can forward traffic arriving from external sources to internal destinations (a.k.a. port forwarding).

In some scenarios it is useful if this port forwarding also works for traffic from internal sources. In this case an internal computer would send traffic to the external IP address of the internet router. The router would then forward this packet back to the internal destination defined by the port forwarding.

Router vendors tend to use their own terms for this (e.g. NAT Reflection or NAT Loopback)

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