code: @Override
  	public Integer getPrefixLength() {
  		Integer ret = cachedPrefixLength;
  		if(ret == null) {
  			int prefix = getMinPrefixLengthForBlock();
  			if(prefix == getBitCount()) {
  				cachedPrefixLength = NO_PREFIX_LENGTH;
  				return null;
  			}
  			return cachedPrefixLength = prefix;
  		}
  		if(ret.intValue() == NO_PREFIX_LENGTH.intValue()) {
  			return null;
  		}
  		return ret;
  	}
docs: @return the number of bits in the prefix.
  
  The prefix is the smallest bit length x for which all possible values with the same first x bits are included in this range of sections,
  unless that value x matches the bit count of this section, in which case the prefix is null.
  
  If the prefix is the OUI bit length (24) then the ODI segments cover all possibly values.