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

Jon_Cisco

32 bits. First 8 are for class A network. Next 19 for for subnets. Last 5 are for hosts.

2^19=524288 subnets.
2^5=32 subtract the network and broadcast addresses 32-2=30 hosts

I know I didn't explain much but this should get you started.

skittledmonkey

Thanks both - I have been struggling with this bit for a while. I actually skipped the subnetting chapters in the Odom book because I didn't want it to confuse the way I learned it on CBT Nuggets, but it doesn't really explain that part of it.

Thanks again - greatly appreciated.

xnx

skittledmonkey wrote: »

Thanks both - I have been struggling with this bit for a while. I actually skipped the subnetting chapters in the Odom book because I didn't want it to confuse the way I learned it on CBT Nuggets, but it doesn't really explain that part of it.

Thanks again - greatly appreciated.

Skipping stuff is no good, get the OCG if you can afford it (get the ICND1 at least) and that book goes through subnetting VERY THOROUGHLY