Quick Subnetting Question

Nytrocide

So I've been using Lammle's IP Subnet Practice Page (IP Subnet Practice Page | Lammle.com) and can get everything correct, but I'm having some issues with some different question types on SubnettingQuestions.com...mainly getting the right number of subnets and hosts correct. Take this example...

Question: How many subnets and hosts per subnet can you get from the network 172.20.0.0/20?

Answer: 16 subnets and 4094 hosts

I can get 16 subnets by using "16" from the /20 and then seeing how many times 16 goes into 256, but I seem to be going blank with the hosts...can someone help dumb ol' me? >_<'

Node Man

with network 172.20.0.0 - 16 bits are used the the network, leaving 16 bits for subnet and hosts. since 4 bits are used by the subnet, then 12 bits are left for hosts.

2^12 = 4096. so 4094 hosts.

Ltat42a

One thing to remember when subnetting, look at what Class address you're subnetting. This is a Class B. The default CIDR for Class B is /16. This question says you have a /20. Then just follow what Node man posted....you'll get it!

newt.chapman

Ltat42a wrote: »

One thing to remember when subnetting, look at what Class address you're subnetting. This is a Class B. The default CIDR for Class B is /16. This question says you have a /20. Then just follow what Node man posted....you'll get it!

To build on this:

Class B is 16 by default. You are given a /20 mask so you need to subtract 16 from 20. Leaving 4 bits to be used for subnets; 2^4 is 16. So thats where the 16 subnets come in to play.

Now you need to find out how many hosts you have per subnet. This is done by taking 32 (total bits in the address) and subtracting your mask from it. So 32 - 20 is 12. 2 ^12 is 4096. Then you take away 2 addresses because they are reserved for the net id and broadcast id. So you are left with 4096-2 = 4094 hosts per subnet.

If you have any questions feel free to ask me!

Nytrocide

Thanks so much! Really helped me a lot...so easy!

One more question...can a CIDR (or subnet) ever be below /8? I've done countless questions on Lammle's (IP Subnet Practice Page) and have never run into below /8. When I typed it in manually it was like it never even subnetted anything...thanks!

boredgamelad

From a purely technical standpoint, yes, you can have a CIDR number below 8. There's a real world reason why you're likely to never see that, though. I'll leave that up to you to figure out.