@Nullable
  static byte[] textToNumericFormatV6(String ipString) {
    // An address can have [2..8] colons, and N colons make N+1 parts.
    String[] parts = ipString.split(":", IPV6_PART_COUNT + 2);
    if (parts.length < 3 || parts.length > IPV6_PART_COUNT + 1) {
      return null;
    }

    // Disregarding the endpoints, find "::" with nothing in between.
    // This indicates that a run of zeroes has been skipped.
    int skipIndex = -1;
    for (int i = 1; i < parts.length - 1; i++) {
      if (parts[i].isEmpty()) {
        if (skipIndex >= 0) {
          return null; // Can't have more than one ::
        }
        skipIndex = i;
      }
    }

    int partsHi; // Number of parts to copy from above/before the "::"
    int partsLo; // Number of parts to copy from below/after the "::"
    if (skipIndex >= 0) {
      // If we found a "::", then check if it also covers the endpoints.
      partsHi = skipIndex;
      partsLo = parts.length - skipIndex - 1;
      if (parts[0].isEmpty() && --partsHi != 0) {
        return null; // ^: requires ^::
      }
      if (parts[parts.length - 1].isEmpty() && --partsLo != 0) {
        return null; // :$ requires ::$
      }
    } else {
      // Otherwise, allocate the entire address to partsHi. The endpoints
      // could still be empty, but parseHextet() will check for that.
      partsHi = parts.length;
      partsLo = 0;
    }

    // If we found a ::, then we must have skipped at least one part.
    // Otherwise, we must have exactly the right number of parts.
    int partsSkipped = IPV6_PART_COUNT - (partsHi + partsLo);
    if (!(skipIndex >= 0 ? partsSkipped >= 1 : partsSkipped == 0)) {
      return null;
    }

    // Now parse the hextets into a byte array.
    ByteBuffer rawBytes = ByteBuffer.allocate(2 * IPV6_PART_COUNT);
    try {
      for (int i = 0; i < partsHi; i++) {
        rawBytes.putShort(parseHextet(parts[i]));
      }
      for (int i = 0; i < partsSkipped; i++) {
        rawBytes.putShort((short) 0);
      }
      for (int i = partsLo; i > 0; i--) {
        rawBytes.putShort(parseHextet(parts[parts.length - i]));
      }
    } catch (NumberFormatException ex) {
      return null;
    }
    return rawBytes.array();
  }
Adapted from code in {@code com.google.common.net.InetAddresses.ipStringToBytes}. This version
separates detection from parsing and checks more carefully about embedded addresses.
zipkin/src/main/java/zipkin2/Endpoint.java