DearAll
Just when I think i have subnetting cracked! I come across a question that throws me, here goes.
An IP subnetting design effort is under way at a company. So far, the senior engineer has decided to use Class B network 172.23.0.0. The design calls for 100 subnets, with the largest subnet needing 500 hosts. Management requires that the design accommodate 50 percent growth in the number of subnets and the size of the largest subnet. The requirements also state that a single mask must be used throughout the Class B network. How many masks meet the requirements?
Explanation
With 50 percent growth, the mask needs to define enough subnet bits to create 150 subnets. As a result, the mask needs at least 8 subnet bits (7 subnet bits supply 27, or 128, subnets, and 8 subnet bits supply 28, or 256, subnets). Similarly, the need for 50 percent growth in the size for the largest subnet means that the host part needs enough bits to number 750 hosts/subnet. Nine host bits are not enough (29 - 2 = 510), but 10 host bits supply 1022 hosts/subnet (210 - 2 = 1022). With 16 network bits existing because of the choice to use a Class B network, the design needs a total of 34 bits (at least) in the mask (16 network, 8 subnet, 10 host), but only 32 bits exist - so no single mask meets the requirements.
Now I get most of the explanation, however I do not understand in the brackets how 7 subnet bits supply 27!!, I get that 2 to the power of 7 = 128 subnets, that is fine with me, but 7 subnet bits supply 27!! also 29-2 = 510 and 210-2 =1022. ok I understand that 2 to the power of 10 = 1024 - 2 =1022, and 2 to the power of 9 = 512 and -2 = 510, but where do the figures 27 and 29 come into it! am I been silly here, please could somebody, make this clear for me, as I do not get it.
Kind regards
Arcaneuk
