Note-:Here, IP address means IP address+subnet mask.

I am studying network design. My question is will there be a moment where you choose IP address for subnetting but turns out finally that it's insufficient(Not in future, just in present).

The question is better worded as "how to choose IP address for subnetting?"

My question would be how would you decide what network id and what host id aka what class of IP address you should choose for subnetting for different amount of hosts? (I know classes are gone, I am just using it for the term host id and network id, and I also know number of hosts are determined by /## term I am just using it for ease to understand).

For eg-: say you have 4 departments including ISP as follows-:

D1=>120 hosts

D2=> 120 hosts

D3=> 120 hosts

ISP=>5 hosts.

So how would you decide which IP address would be enough beforehand? ( English not my first language and I am unable to explain this properly).

Can I choose any IP address? Yes or No? If yes, is any IP address optimal i.e minimum address losses(I am explicitly designing for academia so no future expansion is assumed)?

What's the logic of choosing the optimum minimum loss IP address?

Say I choose a class C ip address( Yes classes are obsolete but I am telling this in terms of network id and host id. It gives 8 bit host address so 256 hosts are possible.but there would be subnet and /## would keep on changing on VLSM.)

Say I choose a class B ip address(in terms of hid and nid), what would happen?

How do I calculate that this is my needs, and this is the class of IP address or "this is the IP address" that I should choose? Or don't I need to calculate this at all? And subnetting is infinite host address possible?

I learnt VLSM from here

Relevant question

Zac67 says

For a private network, just use any subnet(s) from the RFC 1918 ranges, and There's no one-size-fits-all, but for most purposes, /24 subnets are the most reasonable.

He talks about classes being obsolute but doesn't answer how do you choose the optimum IP Address(for just now not thinking about future expansion). So please guide.

  • You can't choose IP addresses/prefixes at random if there's only a remote possibility that you'd connect to the Internet one day. Unless you've bought or leased a range you need to stick to the RFC 1918 prefixes.
    – Zac67
    Jun 23, 2022 at 9:39
  • Also, "VLSM" is an ancient term from the classful era, nearly 30 years ago. Today, it's called CIDR.
    – Zac67
    Jun 23, 2022 at 9:42
  • why did you close the question? i am asking how to choose ip address. in other problem, ip address is given by user. here you have to think of the ip address yourself. i know how to subnet with given ip address.. my question is how i decide to buy an ip address what ip address to choose? you seriously closed this huh? i wasn't mocking you, i was just telling your answer doesn't answer my question. pls give a proper reason for closing this answer. stackexchange answers aren't a good place to learn from scratch as nb here are professional educators. i learnt from that pdf already.
    – el_waleou
    Jun 23, 2022 at 9:53
  • Sorry, your question isn't very clear but the answer you're giving yourself is about subnetting which is a duplicate. Perhaps you can edit your question for more sharpness and I'll be glad to reopen it. You should note that opinion-oriented question or "best practice" are off-topic here, see the help center. You should also note that NE isn't primarily intended for learning (although it can be used that way) but for network professionals to get expert advice.
    – Zac67
    Jun 23, 2022 at 10:25
  • thanks for giving the reason. i know that my question isn't clear but i am not asking here once this confusion came to my mind. i first asked it in 30 forums already(it's been 7+ days since this confusion came to my mind). also i don't ask in forum to learn. teaching isn't to be taken for granted. just like i don't think a professor can build corporate intranet, same way i don't assume any professional knows how to teach so i prefer learning from academics materials/books etc. i have got most of the answers by now by myself though. i found ip add wastage is same in a.b.c.d or w.x.y.z
    – el_waleou
    Jun 23, 2022 at 11:45

1 Answer 1


I think I got my answer. (correct me if this software is faulty).

I will post the details image.

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So you just look at that subnet subnet mask=24

i.e 8 bits for hosts.

i.e total hosts possible is 256. But you count total hosts required ie 294 hosts hence this subnet mask is not allowed.

So can you choose And that would let 512 hosts. I think this is correct answer. Correction very welcomed.

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