Well ARPA already had a margin factror...they had about 256 computers that needed addresses, and left space for 2^24 times 256, or 4,294,967,296 computers. The

Well ARPA already had a margin factror...they had about 256 computers that needed addresses, and left space for 2^24 times 256, or 4,294,967,296 computers. The growth ended up to be exponential over time. So maybe a better margin would be expressed exponentially along the lines of "build it for 2^(doubling_constant * number_of_years) the load."