A NAT router doesn’t know when to remove a UDP mapping – it guesses.

The router simply ages (or times) out the entry when it hasn’t been used for a period of time (usually between 5 and 60 minutes).

With TCP, there’s also a similar aging/timeout to make sure that forgotten or lost sessions don’t pile up, but it’s much longer. Of course, normal TCP connections are properly closed, which means the NAT router can forget about it.

UDP or TCP aging is a trade-off between router resources (fast aging) and compatibility with slow low-bandwidth sessions (slow aging). Sometimes the default settings require tweaking for your workload.

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