Subnetting for Dummies

I'm starting trying to understand subnetting. Any help would be appreciated.

Follow me here:

- Lets say I have one public class C address from my ISP

- Lets call it

- Lets say I have 500 clients.

Why do I even need to subnet? NAT is confusing me... Can I not just have a router with NAT and then do whatever the heck I want on my LAN? (like my two home computers do).

Router WAN ~ Router LAN ~

In the above scenario I can just throw everyone on the 10.x.x.x network and be done? Do I only need to subnet when issues of bandwidth/ collisions come up? I understand all about broadcast/collision domains.. but I'm confused about how subnetting "saves address space" when you can just use NAT to split one address into a million...

My head hurts... if anyone can point me in the right direction without using any big words it would be much appreciated....

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You cannot make a network with 500 clinets in class c address if you want all clients to be in the same subnet, because class C can hold

253 clients. You have to left one address to represent the network (, one for representing interface (example and the last address int the subnet is a broadcast addres s( So if you want to make network for 500 clinets you have to use class B address (example: If you use this addres without the subnet you can have 255*255 clients in the same subnet. If you want to preserve address space then you use subnet and you use because with this subnet you have 2*255 and that is enough for your 500 clients. And if you have one public ip address you have to use subnet just to make all clients in one subnet, bu you cannot use nat because nat can make from one private address to one public. You have to use pat,and all 500 clients can go out on the same address and the router will know from which address it came, because router for every address use different port.


Dennis je napisao/la:

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I think you misread? Dont confuse me more! :) In the above scenario, my LAN network was using class A, mask

I understand that. But I dont why would I need to "preserve address space" on my LAN (using the above scenario)....? Im starting with a class A address of are there not endless possibilities and combinations of subnets I could use? This is where I am a bit foggy.

Explain please? On my home lan my router uses NAT to split my one public IP address from my ISP into countless local IP addresses. What is this 1:1 relationship you speak of?

Hmm... I'm afraid to ask.. but what is PAT???

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Well, you don't actually HAVE TO subnet and preserve address space. But, you should keep an eye on the future and the bigger picture. Suppose your company grows and you end up with 30 locations. Now, you have 30 subnets that are needed (plus wan links, etc...) Well, okay, you can tap into the

172 and 192 address. But, if you had the same short sighted attitude, they would run out quickly as well. Also, as you grow, you will want to consider address summarization. When you don't consider your address plan ahead of time, this will become a cumbersome process and you will end up needed to renumber at that time. As they say, a stitch in time...

Yes, there are endless (well, virtually endless) possibilites using the 10 network. But, you need the forsight to consider that ahead of time. What I've typically done is reserve one class C size address per location (,, etc...). If the site requires more address, you can consider a larger address block (

Also, consider if you want different DMZ blocks within the Class C that the ISP assigned. You need to subnet that block into the appropriate number of subnets/hosts and apply to a multiport firewall.

So, if you don't have the need at this point, there is nothing that will force you to. But, it is good practice.

PAT is really what your home router is doing. NAT translates one outside address to one inside address, thus the 1:1 reference. Your home router will dynamically assign a port to you when you make an outside request, not an address, since it only has one to share. That is why it is Port Address Translation.

Hope that helps,


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Dennis je napisao/la:


You dont't have to do it, it can be done without subnet, but if you learn subnetting it will help you in the future, when you wouuld be dealing with large networks, and then you will have to be carefull. But you're right, class A has many many possibilities and if you use it for only one LAN, don't be bother with subnet :)

PAT is NAT with overload. It means that it uses differnet ports for every ip address in your LAN whet it translate private in public. With PAT you can in theory translate 60000 ip addresses but the real number is around 4 000

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Microsoft calls PAT "Internet Connection Sharnig".

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Great posts guys thanks for info! Everything is starting to make sense now....

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With the above netmask of (actually /22) you'll have 4*255-3 addresses. The desired 2*255-3 would be (/23)



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