Subnetting help

skittledmonkey

Hello,

I have pretty much got subnetting nailed down but I struggle working out one aspect. When you are working out how many hosts and subnets you can create from a particular ip and mask I can get the hosts, but not sure how to work out the subnet - could someone please help explain?

For example:

• How many hosts could you put on each subnet of 12.0.0.0 if you used a mask of 255.255.255.224? How many subnets would you have created?
  • ? 30 useable hosts. 524,288 subnets.
  • ? 32 useable hosts. 524,288 subnets.
  • ? 30 useable hosts. 262,144 subnets.
  • ? 32 useable hosts. 2048 subnets.
• I can see it will be 30 useable hosts, but how do you get the subnets?

EdTheLad

256 - 224 = 32; -2 for subnet and broadcast = 30

So how many subnets, well first you look at the first octet and figure out what the natural mask will be i.e. classful mask
1-126 range means class A, class A has 8 bit mask.

Now above we found the host portion was 32 bits i.e. 2^5 = 32 ; so we used 5 bits for hosts

classful mask = 8
host bits = 5
5 + 8 =13

ip address = 32 bits
32 - 13 = 19 bits for subneting

2^19 = 524288