Why Do You Subnet?

Why? The Golden Question

All over the internet you will find information on ‘How to Subnet’.   Suddenly you are thrown into a world of bits and binary and given charts of ‘How to Subnet’.   But why do you subnet?

Subnets are created to separate areas of your network for security and/or to hold down broadcasts.  Computers constantly ‘talk’ to each other.   If you have a network of 10 computers, the talking (opening files or programs) or broadcasting packets that are sent out during networking is not much traffic at all.   However if you have 10,000 computers talking and passing data, you will have thousands of computers passing data and your network will slow down from all of the information.   If you still don’t understand, invite 10 friends over for a get together.  It’ll get noisy but you can still hear every conversation.    Now, go to a rock concert with 10,000 screaming fans.   Get the picture.

Subnetting allows you to divide areas of your network out to prevent this.   So here comes the golden question.  How can you get them to ‘talk’ when you need to?   With the installation of a Layer 3 Switch or a router, these subnets can talk.

And for all of the folks who know a ton of details on the subject, this is just an example for people who are searching why to subnet.

Other Ways to Divide Your Network

Are there other ways to divide your network?  You can use VLANs.  Devices such as switches allow virtual lans to be set up.    By purchasing managed switches, you can separate different areas of your network thus separating areas for security and holding down traffic.

So now I know why – but how and don’t give me binary and all the fuzzy stuff!

The best explaination I have found is at this link.   A simple chart with examples!

Here’s my example -

Subnetting allows your network to become more efficient and allows you to separate your network by subnets.

Let’s say your network is on a 192.168.1.x numbering scheme.

How do you know what your network is?  Open a command prompt and type ipconfig.   See the gateway?  That is your starting number for your network.   The network allows 254 computers to be in the broadcast domain.

So if you do an ipconfig and see this-

192.168.1.125      - your ip address
255.255.255.0    - your current subnet
192.168.1.1           – your gateway (This is your router or firewall – the point leading to the internet)

Now let’s say you have 20 devices in your network.   How can you effectively hold down broadcast and prevent your router from building tables for each device?  How can you make it more effective and hold down traffic?   Actually without understanding binary, you can do this.   Remember this is basics and should help you understand.

You see under the current network, you can have 254 devices in your network.  Printers, routers, computers, tablets, netbooks, notebooks, etc.    The last set of numbers called an octet only goes from 1-254.   But lets narrow this down.

You can use a CIDR calculator online to determine how to tweak your network.   A good site is Subnet Calculator.   Go there and enter your router’s number in the ip field.   Now go down and enter a number close to the number of devices you have on your network.  In this example, enter 30.

Did you see where the subnet changed to 255.255.255.224?    (Did you notice that 224 + 30 is 254?)

Ok, now open your adapter settings by clicking the network icon at the bottom right corner and selecting Open Network and Sharing Center.

Now click on the adapter settings.

Right click and select properties on your adapter that is active.

Now select Internet Protocol v4

Enter an ip address statically (manually put it in).  Put in your new subnet and your gateway.   You must also put in your DNS or a public DNS server.  You should keep a list of each devices number to avoid any problems.  All ip addresses must be different on each device.

The only valid numbers in this broadcast domain now is – 192.168.1.1, 192.168.1.2, 192.168.1.3 continuing until you get to 192.168.1.30 (remember you chose to have up to 30 devices on your network.)  If you enter any number above this, you will get an error.

Error screen showing an ip address above what you selected.

If you get the above error screen, you have entered an invalid number.

After you select a number between 192.168.1.1 and 192.168.1.30, click OK and exit all screens by clicking OK.

If you do an ipconfig now, you will see this-

When you change subnets as the above example shows, you have to remember that you may lose connectivity to devices such as routers or network printers.   You will need to set devices such as servers and these devices on the same subnet so that whoever is assigned this subnet can communicate with them.

This by no means is a thorough subnetting lesson.   It can become much more complex by introducing multiple routers into your network, DHCP servers or other devices.   This is a simple how-to that shows you how broadcast domains can be more efficient.

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4 comments on “Why Do You Subnet?

  1. Jon Ward says:

    Here’s some experience talking. That’s a nice link, btw. Subnetting can get very mathematical and complicated. On the networks that I work with, we subnet for security. We have some hosts on subnets by themselves for security purposes, we have some hosts on subnets for organizational purposes(like requiring different OUs), we have some hosts on subnets for broadcast minimization.

    Why do we use subnets instead of just putting them on different class 3 networks? It’s complicated, sort of. Some of our hosts require a subnet that can only support a single host for security purposes. That throws off the balance of using JUST different class 3 networks. Why not use a whole standard class 3 subnet and only put this host on it? Because other hosts CAN join it, enabling a rogue host to connect and compromise the network. Why don’t we just buy a switch and set up the subnet on that and use class 3 networks for other hosts that don’t require subnets? It costs money and we’re knowledgable and not lazy. The team I work with can setup subnets in their sleep. We can calculate the CIDR in our heads and we don’t think twice about it.

    Before I started this job, I thought: “Why the hell does anyone use subnets anymore, just use NAT!” I now sing a different song. Security, budgets, and pride keep us in the clear. Does it make things more complicated? Not really, once it’s set up, it’s done and we really don’t have to do much else with it. Yeah, we have to consider it when forming new subnets on that router but that’s no big deal when you’re at this level.

    No doubt, subnetting is big boy(or girl, for all you PC people), but if you’re not one of the big dogs, get off the porch.

  2. Ramblinrick says:

    Great post… You just gave me an education!

    Rick

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