does sombody have an easy to remmber formula to resolve the questions that as how many subnets and hosts are available from this specified network address/mask

eg ...

Q: You have subnetted a network address (213.105.72.0) with a /28 mask.. How many usable subnetworks and usable hosts (on each subnetwork are avaialble)

A: 14 networks and 14 hosts per net


I just need an easy way to work these ones out

cheers
Gary

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First, determine what the default class of the address is and what the default subnet mask is for that class of address. In this instance, it is a class C address with a default mask of 255.255.255.0 or /24. Then determine how many bits have been used for subnetting. Since the default mask is /24 and the mask in the question is /28, you know that they have borrowed four bits from the host portion of the address to use for subnetting. So, you now have four bits for subnetting and four bits for host addresses. Now, use the formula ((2^x) - 2) to determine the number of available hosts or subnets, where x represents the number of bits you have to work with. In this instance, you have four subnet bits to work with, so you use the formula 2 to the power of four (16) minus 2 = 14. Since you also have four bits for host addresses, the results of the formula are the same.

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bit
value -2 128 64 32 16 8 4 2 1
subnet
mask 192 224 240 248 252 254
# of
subnets 2 6 14 30 62 126
# of
bits 25 26 27 28 29 30 31

Okay, using your subnetting question, I look up /28 on my chart, see it has a subnet mask of .240, and in a class C, that shows 16-2 for # of hosts, and 14 for number of subnets.

You can memorize this char easy. Starting from the top count the bit values from the left most bit 12
then on the 2nd row, add the bit value above, so for the first entry 128+64 = 192
Then the 3rd row start with 2, and add the number plus 2
Then the forth row, is just the number of bits used in the mask.

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