code: @Override
  	public Integer getPrefixLengthForSingleBlock() {
  		int count = getDivisionCount();
  		int totalPrefix = 0;
  		for(int i = 0; i < count; i++) {
  			AddressDivisionBase div = getDivision(i);
  			Integer divPrefix = div.getPrefixLengthForSingleBlock();
  			if(divPrefix == null) {
  				return null;
  			}
  			totalPrefix += divPrefix;
  			if(divPrefix < div.getBitCount()) {
  				//remaining segments must be full range or we return null
  				for(i++; i < count; i++) {
  					AddressDivisionBase laterDiv = getDivision(i);
  					if(!laterDiv.isFullRange()) {
  						return null;
  					}
  				}
  			}
  		}
  		return cacheBits(totalPrefix);
  	}
docs: Returns a prefix length for which the range of this segment grouping matches the the block of addresses for that prefix.
  
  If no such prefix exists, returns null
  
  If this segment grouping represents a single value, returns the bit length
  
  @return the prefix length or null