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I am learning the basics of subnetting however am faced with a dilemma on approaching the issue. For example I have been tasked to set up a network with 20 hosts using a Class C network addressing scheme which I understand to be with the default subnet mask of Now I understand that it leaves me with 254 hosts 0 being the subnet and 255 being the broadcast.

So if I have 20 hosts to setup that leaves 234 host spaces wasted unless the network grows to include that many hosts. Based on the requirements of 20 hosts I have the following questions.

  1. How do I accommodate future growth e.g. how many hosts do I allow for?
  2. If I were to simply restrict the number of hosts to 20 I take it that would leave me with a /27 subnet i.e. Is that right?
  3. If 2 point is correct I would have 8 subnets with the following network subnet address, broadcast addess and starting and ending host address. Is that right?
  4. If 3 is right do I still a router to allow each subnet to communicate with one another?

Subnet addresses

The broadcast address of each of these would be

The starting and ending host addresses for each of these would be - - - -

Unless I have understood it incorrectly my host addresses should be as follows as opposed to what I thought they were above  - - - - - -
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The fact you refer to it as a Class C network addressing schemes tells me this is homework.... – Not Kyle stop stalking me Oct 25 '11 at 21:24
+1 for well-done homework – Jens Erat Oct 25 '11 at 21:27
@Kyle - Actually no. This is a real network deployment I have been asked to implement as part of my internship – PeanutsMonkey Oct 25 '11 at 21:54
It's not wrong, but this classification is somehow outdated and used in academic context, because you're able to divide in other subnetwork sizes than these classes. See you /27-network above! – Jens Erat Oct 25 '11 at 22:58
@PeanutsMonkey Nothing in particular is wrong with the phrasing. It's just very rare (I have never seen it) for someone refer to a network as class c outside of a school since classless inter-domain routing was introduced in 93 over 15 years ago... – Not Kyle stop stalking me Oct 26 '11 at 12:31
up vote 2 down vote accepted
  1. You're totally right, 254 hosts are allowed, so you can add 234.
  2. Exactly, /28 would be too small, /27 fits fine (30 hosts allowed)
  3. Exactly.
  4. If you have a router, it can route traffic between the subnets (if they're connected of course and the router knows how to route between them).
share|improve this answer
Thanks Ranon. So my subnets, broadcast and host addresses are correct? When you say route traffic between the subnets, I take it I don't require to have anything else as they are on the same network i.e. 192.168.*.* unlike if the network was say a different network such as 192.168.1.*? Is that right? Now if I wanted to have the 192.168.*.* network communicate with 192.168.1.*, what would I need to do? – PeanutsMonkey Oct 25 '11 at 21:49
How do I also accommodate a larger number of hosts for the future if the requirement now is only 20? – PeanutsMonkey Oct 25 '11 at 21:54
Your answer in 4 reads if I have a router, what if I don't have a router, I take it a switch would suffice? If I do have a router, I take it I would either need a router with 8 ports or a single router with a switch with 8 ports? Is that correct? – PeanutsMonkey Oct 25 '11 at 22:00
No, a switch (at least the devices usually called switch) wouldn't suffice. You need a router (that is sometimes called "layer 3 switch") which knows about the other networks. A switch can only pass packets to other computers in the same subnet, have a look at wikipedia which has a really good explanation. – Jens Erat Oct 25 '11 at 22:51
For having some free IPs for new hosts left, estimate how much that could get. Think about possible splits into multiple subnets if your networks grows really much. That depends on your situation and cannot be judged without detailed knowledge only you (or even your boss) can have. – Jens Erat Oct 25 '11 at 23:10

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