Subnetting:

What does it do?

Subnetting divises a network into sub-networks.

Why?

Because although your ip class might allow tens of thousands of possible hosts, a standard ethernet segment can only cope with about 1000.

It's cool

Theory:

Ok, so assuming you know how internal ip addressing works (e.g classes, host octets and network octets) then we can skip wading through all of that crap. If you don't know that stuff, look at suggested reading.

Ok then, so we know that computers don't see ips as decimal numbers like we do (e.g 172.16.80.0), nope it sees that in binary! So for the computer to tell the network octets and the host octets apart, it uses something called a subnet mask .

Each ip class has a default subnet mask, so a class B ip (e.g 172.16.80.0) will have a default subnet mask of 255.255.0.0. This is because the subnet mask uses the first 16 bits to identify the network.

In binary this would look like:

CODE :


This kind of address can also be noted as /16, so 16 bits identifies the network portion of this ip address.

Right so that's default subnet masks sorted right? Well sometimes we have to go beyond defaults and start customizing them!

Why?

Well if you have a regular Class B address you'll have 65,534 addresses available to you, that's great right? Won't need any more than that?

Wrong!

Unfortunately a standard network segment can only support roughly 1,000 network hosts. That's not very efficient!

So with a class B network for instance, I don't want to waste addresses but I want to stay a Class B network!

What do I do? I subnet!! Yes that's right, I use the following:

CODE :


I have one physical network but am using several logical sub-networks (subnets).

I have now borrowed bits from my host portion. So when I create a subnet mask I'm going to tell the computer which bits to pay attention to, so it can identify the network.

In this case I borrowed 8 bits, an octet, to identify the network.

So since I've done this, I'm going to have to change the subnet mask, so the poor computer can understand what's going on.

This is how they're created:

It's all a case of how many bits I borrow, borrow 1 bit and then my subnet mask becomes 255.255.128.0.

Confused? I hope not! Here's a table to figure it out:



So 172.16.80.0/20 means there are 16 bits (Class B ip) and 4 subnet bits, therefore the subnet mask would be: 255.255.240.0

Ok let's take another look at that

CODE :


Note: Until the width gets enlarged please remember that the host and network portions are 2 octets (16 bytes) in length.

We have actually used a technique called AND'ing, a rather simple binary operation:

CODE :

This gives us the result of

CODE :


See how that works? Good.

Calculating the number of possible subnets:

Now you should know a magic formula which is 2N-2 If you don't, read up on networking.

We can use that same rule to calculate the number of possible subnets. N is the amount of bits you borrowed from the default host portion.

Now, RFC 150 reccomend that subnet zero and all one's subnets should not be used for addressing and that's why we have -2.

However soon this will not be neccesary, in fact with Cisco devices it's already obsolete in the advent of IOS 2.0.

So anyway, calculating possible subnets. Just do 2N-2 and N is the amount of bits you borrowed from the default host portion.

So if you borrowed one octet you'd get 256 possible subnets and then -2, 254!

Conclusion:

This was a basic overview of subnetting, if more detailed articles are wanted I'll write them. Thanks for reading!

-sas01