Subnetting - usable addresses

urstuffplz1

Hi all,

I have an assignment for my university course and one of the questions has asked:

150.16.0.0 / 18

How many subnets and usable subnets are there?

I am lead to believe that there is 4 subnets (10, 11, 01, 00 binary) and all are usable. My friend believes that only 2 are usable and has found sources online stating this, with 2 being unusable.

Can someone just confirm please, and if I am wrong, just explain why?

Kind regards,

Stephen

linuxabuser

You have to be more specific. You determine the size of the subnet you want by how many hosts need to fit in it.

That /18 could be one big usable subnet. It could also be a bunch of /30s.

There has to be more information to answer the question.

kohr-ah

It really depends.
Is your professor counting that you get to break it down further via CIDR?
Because in that case I can take you /18 and divide it up into /30s and get WAY over 4 subnets.

If you are talking basics (Like lets not break it down) then yes 4 are usable. It is called IP Subnet Zero.

150.16.0.1 - 150.16.63.254
150.16.64.1 - 150.16.127.254
150.16.128.1 - 150.16.191.254
150.16.192.1 - 150.16.255.254

Each one gets 16,382 usable hosts.(There is a total of 16,834 addresses but remember to use the 2^n-2 rule = 16382)

OctalDump

I still don't understand the question. If it is related to Subnet Zero, I'd refer you to RFC 1878 which obsoleted subnet zero more than 20 years ago.

So, for 150.16.0.0/18 it could be divided into many (4096) /30 subnets with 2 usable addresses each, or into two /19 networks with 8190 usable addresses.

Or is the question asking for network 150.16.0.0, how many /18 subnets can you create? Since 150.16.0.0 is a class B, it's /16, which allows 4 /18 sub-networks, as kohr-ah describes.

urstuffplz1

Hi all,

Thank you for all the replies.

It was just asking how many subnets for that range, not subnetting further. The CIDR has to remain /18 for the 150.16.0.0 address.

I am write in my thinking that for this CIDR and address you can have 4 subnets, each with 16,382 usable hosts in each.

Stephen

d4nz1g

If he considers that 150.16.0.0 is a class B address, where 150.16 is the network portion and 0.0 (low order 16 bits) are the host portion. Therefore, the mask for this class is /16.

In the range 150.16.0.0/18, you will keep the 150.16 network portion with 16 bits, and 2 bits for the subnet portion. Remaining only 14 for the host portion.

So, as OctalDump mentioned: Since 150.16.0.0 is a class B, it's /16, which allows 4 /18 sub-networks

vasyvasy

urstuffplz1 wrote: »

The CIDR has to remain /18 for the 150.16.0.0 address.
Stephen

There is no such thing as subnetting, while leaving the SM the same