Source NAT
also called SNAT, Masquerading
Rewriting the source address of outbound packets so that many private addresses share one public address, with a translation table mapping replies back.
The mechanism behind a NAT gateway. Each outbound connection is assigned a source port on the shared public address, recorded in a translation table, and replies are matched back by that port.
Two properties follow. Outbound connections work; inbound connections cannot be initiated, because there is no table entry to match — which is why a private subnet is unreachable from outside and why NAT is often mistaken for a security control (it is a side effect, not a designed one).
And the table is finite. Each destination address and port pair consumes an entry, so a workload making very many concurrent connections to the same destination can exhaust the available source ports — producing connection failures that look like the destination refusing you.