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Determine whether the address is a loopback address.
bool is_loopback() const;
          This function tests whether the address is in the address block 127.0.0.0/8, which
          corresponds to the address range 127.0.0.0 -
          127.255.255.255.
        
          (to_uint() & 0xFF000000) == 0x7F000000.