A special address (such as 127.0.0.1) that refers to the device itself.
A special IP address range used to test communication within the same host, referring to the device itself. Since it's always active regardless of physical NIC status, it's also commonly used as a router's ID address. IPv4 reserves the entire 127.0.0.0/8 range for this, while IPv6 assigns just the single address ::1.
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