# Subnetting help

Member Posts: 25 ■□□□□□□□□□
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?

## Comments

• Member Posts: 2,111 ■■■■□□□□□□
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
Networking, sometimes i love it, mostly i hate it.Its all about the \$\$\$\$
• Member Posts: 1,772 ■■■■■■■■□□
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.
• Member Posts: 25 ■□□□□□□□□□
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.
• Do they matter? UKMember Posts: 464 ■■■□□□□□□□
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
Getting There ...

Lab Equipment: Using Cisco CSRs and 4 Switches currently
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