code: public static IPv6Network fromString(String string)
      {
          if (string.indexOf('/') == -1)
          {
              throw new IllegalArgumentException("Expected format is network-address/prefix-length");
          }
  
          final String networkAddressString = parseNetworkAddress(string);
          int prefixLength = parsePrefixLength(string);
  
          final IPv6Address networkAddress = IPv6Address.fromString(networkAddressString);
  
          return fromAddressAndMask(networkAddress, new IPv6NetworkMask(prefixLength));
      }
docs: Create an IPv6 network from its String representation. For example "1234:5678:abcd:0:0:0:0:0/64" or "2001::ff/128".
  
  @param string string representation
  @return ipv6 network