Subnet mask for a range of hosts

[Mokhalil82]

Hi

I am looking to include the following range of hosts into a single subnet, 10.10.20.1 - 10.10.26.254 just for summary purposes.

That gives me 2046 hosts so I need a /21 subnet. So I am looking to use 10.10.20.0/21.

However when I check the range 10.10.20.0/21 on an online subnet calculator, it provides the range from 10.10.16.1 - 10.10.23.254

Am I correct or is the online calculator? I understand I can start from the beginning of the range but those addresses are in use.

TIA

[luis_cordova · Accepted solution]

Hi @Mokhalil82 ,

If you take it to bits you understand it better:

Network: 10.10.0.0

Hosts: 2046

Operation: 2^11= 2048

We take 11 bits of the host portion

10.10.0000 0|000.0000 0000

Your problem is that to make the network address 10.10.20.0 occupies bits of the host portion, which is not possible.

Somes  possible networks with a mask /21 are:

10.10.8.0 - 10.10.15.255

10.10.16.0 - 10.10.23.255

10.10.24.0 - 10.10.31.255

10.10.32.0 - 10.10.39.255

10.10.40.0 - 10.10.47.255

Regards

[Richard Burts · Accepted solution]

There are several aspects that perhaps make this an interesting discussion. The original poster tells us that "I am looking to include the following range of hosts into a single subnet, 10.10.20.1 - 10.10.26.254".  And the original poster determines that for this range of addresses he needs 2046 addresses. So far so good. The OP then decides that to get this number of addresses he should have a /21 subnet. Unfortunately as @luis_cordova demonstrates this does not work. 10.10.20.0/21 includes only some but not all of the address range required. To achieve the requirement the OP will need a /20 subnet.

So there are several lessons to be learned from this discussion:

- when summarizing the subnet always begins in binary where the 1s turn into 0s. So be aware of where the bit boundary occurs in your address range.

- when planning the IP addressing for a network be aware of the bit boundary and of the number of addresses needed. If the network addressing for this part of the network had started at 10.10.16.0 it would have been easy to accommodate a /21 summarization.

- when summarizing be aware of the implications of claiming addresses in the summary that are not actually in the part of the network you are configuring. If you configure 10.10.20.0/21 be careful whether 10.10.16 or 10.10.17 or 10.10.18 or 10.10.19 do exist somewhere else in the network.

- when planning summarization and the current addressing does not quite match summarizing to a single summary, then consider summarization using several smaller summary addresses. Perhaps 10.10.20.0/22 and 10.10.24.0/23 and 10.10.26.0/24.

HTH

Rick

[Mokhalil82]

Thankyou for the responses. 

What if I already have /24 and /23 subnets already in the range

Ie 10.10.0.0/23, 10.10.2.0/23 and the most recent being 10.10.18.0/23 etc.

But now at 10.10.20.0 i need more hosts in the single subnet, i cant see why 10.10.20.0/21 will not cover me to 10.10.26.254.

I know the best practice is to plan and subnet the larger networks first, but will taking subnets at random points in a major range not work. 

I am sure this is how i have seen subnetting in real life scenarios.

[luis_cordova]

Hi @Mokhalil82 ,

Q: But now at 10.10.20.0 i need more hosts in the single subnet, i cant see why 10.10.20.0/21 will not cover me to 10.10.26.254.

A: To understand it, let's go back to bits.

The network that you propose would be something like this:

10.10.20.0/21 = 10.10.0001 0|100.0000 0000

As you can see, the decimal 20 in the third octet occupies one bit within the host portion of the network 10.10.16.0/21.

Therefore, the network you propose is not possible

I suggest you occupy one of the networks that I proposed in the previous answer.

For example, the network 10.10.24.0/21 It accommodates your requirements and does not run into your existing networks.

Network: 10.10.24.0

Hosts: 10.10.24.1 - 10.10.31.254

Broadcast: 10.10.31.255

Regards

[Mokhalil82]

Thankyou, now i understand why, and it has also been a refresher of basic subnetting concepts