In each version of IP, certain special-purpose addresses are defined. One of these that is worth knowing is the loopback address, which is always assigned to a special loopback interface, a virtual device that simply echoes transmitted packets right back to the sender. The loopback interface is very useful for testing, because packets sent to that address are immediately returned to the destination. Moreover, it is present on every host and can be used even when a computer has no other interfaces (i.e., is not connected to the network). The loopback address for IPv4 is 127.0.0.1;1 for IPv6 it is 0:0:0:0:0:0:0:1 (or just ::1...
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