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When the client and server are on the same computer, the networking stack uses an optimization (or shortcut) and the TCP or UDP traffic does not go down the stack to the NIC but is directly routed to the receiving application. This is often termed loopback traffic.
This means that applications such as Wireshark, using a capture driver of npcap or WinPcap, that (effectively) capture just above the NIC, don't see the traffic.
A loopback adaptor allows a capturing application to see the loopback traffic.
