code: private static CharSequence convertReverseDNSIPv4(String str, int suffixStartIndex) throws AddressStringException {
  		StringBuilder builder = new StringBuilder(suffixStartIndex);
  		int segCount = 0;
  		int j = suffixStartIndex;
  		for(int i = suffixStartIndex - 1; i > 0; i--) {
  			char c1 = str.charAt(i);
  			if(c1 == IPv4Address.SEGMENT_SEPARATOR) {
  				if(j - i <= 1) {
  					throw new AddressStringException(str, i);
  				}
  				for(int k = i + 1; k < j; k++) {
  					builder.append(str.charAt(k));
  				}
  				builder.append(c1);
  				j = i;
  				segCount++;
  			}
  		}
  		for(int k = 0; k < j; k++) {
  			builder.append(str.charAt(k));
  		}
  		if(segCount + 1 != IPv4Address.SEGMENT_COUNT) {
  			throw new AddressStringException(str, 0);
  		}
  		return builder;
  	}
docs: 123.2.3.4 is 4.3.2.123.in-addr.arpa.