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Determine whether the address is a multicast address.
bool is_multicast() const;
          This function tests whether the address is in the multicast address block
          224.0.0.0/4,
          which corresponds to the address range 224.0.0.0 -
          239.255.255.255.
        
          (to_uint() & 0xF0000000) == 0xE0000000.