# parseIPv6Address method

List<int> parseIPv6Address (String host, [ int start = 0, int end ])

Parse the `host` as an IP version 6 (IPv6) address, returning the address as a list of 16 bytes in network byte order (big endian).

Throws a FormatException if `host` is not a valid IPv6 address representation.

Acts on the substring from `start` to `end`. If `end` is omitted, it defaults ot the end of the string.

Some examples of IPv6 addresses:

- ::1
- FEDC:BA98:7654:3210:FEDC:BA98:7654:3210
- 3ffe:2a00💯7031::1
- ::FFFF:129.144.52.38
- 2010:836B:4179::836B:4179

## Implementation

    static List<int> parseIPv6Address(String host, [int start = 0, int end]) {
      end ??= host.length;
      // An IPv6 address consists of exactly 8 parts of 1-4 hex digits, separated
      // by `:`'s, with the following exceptions:
      //
      //  - One (and only one) wildcard (`::`) may be present, representing a fill
      //    of 0's. The IPv6 `::` is thus 16 bytes of `0`.
      //  - The last two parts may be replaced by an IPv4 "dotted-quad" address.

      // Helper function for reporting a badly formatted IPv6 address.
      void error(String msg, [position]) {
        throw FormatException('Illegal IPv6 address, $msg', host, position);
      }

      // Parse a hex block.
      int parseHex(int start, int end) {
        if (end - start > 4) {
          error('an IPv6 part can only contain a maximum of 4 hex digits', start);
        }
        int value = int.parse(host.substring(start, end), radix: 16);
        if (value < 0 || value > 0xFFFF) {
          error('each part must be in the range of `0x0..0xFFFF`', start);
        }
        return value;
      }

      if (host.length < 2) error('address is too short');
      List<int> parts = [];
      bool wildcardSeen = false;
      // Set if seeing a ".", suggesting that there is an IPv4 address.
      bool seenDot = false;
      int partStart = start;
      // Parse all parts, except a potential last one.
      for (int i = start; i < end; i++) {
        int char = host.codeUnitAt(i);
        if (char == _COLON) {
          if (i == start) {
            // If we see a `:` in the beginning, expect wildcard.
            i++;
            if (host.codeUnitAt(i) != _COLON) {
              error('invalid start colon.', i);
            }
            partStart = i;
          }
          if (i == partStart) {
            // Wildcard. We only allow one.
            if (wildcardSeen) {
              error('only one wildcard `::` is allowed', i);
            }
            wildcardSeen = true;
            parts.add(-1);
          } else {
            // Found a single colon. Parse [partStart..i] as a hex entry.
            parts.add(parseHex(partStart, i));
          }
          partStart = i + 1;
        } else if (char == _DOT) {
          seenDot = true;
        }
      }
      if (parts.length == 0) error('too few parts');
      bool atEnd = (partStart == end);
      bool isLastWildcard = (parts.last == -1);
      if (atEnd && !isLastWildcard) {
        error('expected a part after last `:`', end);
      }
      if (!atEnd) {
        if (!seenDot) {
          parts.add(parseHex(partStart, end));
        } else {
          List<int> last = _parseIPv4Address(host, partStart, end);
          parts.add(last[0] << 8 | last[1]);
          parts.add(last[2] << 8 | last[3]);
        }
      }
      if (wildcardSeen) {
        if (parts.length > 7) {
          error('an address with a wildcard must have less than 7 parts');
        }
      } else if (parts.length != 8) {
        error('an address without a wildcard must contain exactly 8 parts');
      }
      List<int> bytes = Uint8List(16);
      for (int i = 0, index = 0; i < parts.length; i++) {
        int value = parts[i];
        if (value == -1) {
          int wildCardLength = 9 - parts.length;
          for (int j = 0; j < wildCardLength; j++) {
            bytes[index] = 0;
            bytes[index + 1] = 0;
            index += 2;
          }
        } else {
          bytes[index] = value >> 8;
          bytes[index + 1] = value & 0xff;
          index += 2;
        }
      }
      return bytes;
    }