1. Subnetting IP4
ICND/CCNA Prep

2. What is subnetting and why do I care?
Basically, subnetting is the process of taking one network and dividing it into smaller networks. For example, if we have a /16 network, this gives us 65,536 (2^16) possible hosts on the network. But, we could divide this up into two networks with 32,768 hosts each. More on this in a bit…
For your ICND and CCNA exams, you need to be able to do this in your head, very quickly. I assure you that while you may not get exam questions that ask you to directly subnet at network (like we did above), you will be given information about a network and have to figure out what’s wrong with it – and that will require that you can subnet quickly.

3. Baby Steps: Counting in Binary.
The first thing you need to do is be able to count in binary and know your powers of two (up to 2^8). See, an IP address looks like this:
But in binary it looks like this:
So how did we get there? The first thing is to look at these octets (each portion of the IP separated by a “.”) as individual numbers. Don’t worry, we’ll put it all together later. First though, our powers of 2:
2^0 = 1 2^1 = 2 2^2 = 4 2^3 = 8 2^4 = ^5 = ^6 = ^7 = 128

4. More on the powers of 2 So right now we know:
2^0 = 1 2^1 = 2 2^2 = 4 2^3 = 8 2^4 = ^5 = ^6 = ^7 = 128
The first section of our IP address is “172”. Our binary representation of that was Let’s tear that apart:
Powers of 2: 2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^ in Binary: Result:
Sum of Results: = 172

5. More on the powers of 2
If you’re like me, you’re already sick of this, but you need to know it, so let’s tear down the rest of that address ( ). On the previous slide, we did 172 – now let’s do the rest:
Result Powers of 2: 2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0 16 in Binary: Result:
Result Powers of 2: 2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0 16 in Binary: Result:
Result Powers of 2: 2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0 16 in Binary: Result:

6. Recap on powers of 2
Powers of two are key in subnetting. The good news is, you only need to know the first 8 (2^0 through 2^7), and they are 1, 2, 4, 8, 16, 32, 64, and 128
Now, a bit of practice of what we’ve done so far:
Powers of 2:
2^4 = 16 = 2^ 2^0 =
128 = 2^ 32 = 2^ 2^6 =
Convert to Binary
Convert to an IP Address = = = = = = = =

7. Back to subnetting
Ok, we can count by twos now – good game. Now that those basics are out of the way, let’s see how we can take a network address – which could have been assigned to us from a network administrator or an ISP – and divide into multiple, smaller networks, a.k.a sub-networks, a.k.a. “subnet it.”
Example:
We’re given the classful (c-class) network address of /24 and need to subnet it. First, we need to recognize that we can’t make the “network” that has already been given to us (by whomever) bigger, we can only make it smaller. That means that the /24 (known as CIDR notation), which defines the network portion of the address that was assigned to you, is “fixed” when looking to the left. That /24 means the first 24 bits ( ) are unchangeable. However, we have the last 8 bits that we can play with. I know that’s wordy – for now, just accept that the part won’t be changed, ever…

8. Subnetting
We have our /24 network. We know the part is locked. We also know that this is denoted by the /24, meaning the first 24 bits (the first 3 octects – remember, in binary those are groups of eight 1s and 0s and 3x8=24) will never change. But now we’re the network admin, and we want to dish out network addresses to two junior admins. How do we do this? Simple, we steal 1 more bit, this turns our /24 into a /25. Remember, the first 24 (to the left) are locked, but we can play with the stuff to the right….
….let’s look at this in binary:
/24 =
First 24 bits, locked
Last 8 bits.
We can play with these.

9. Subnetting From out last slide:
/24 =
Let’s turn this into two networks. To do that, we’re going to steal 1 bit from the last 8 bits. This gives us 2 networks (2^1 – told you powers of 2 were important)
The one bit we’re stealing from the last octet does three things:
1) It means we’re using a /25 mask now (24 + the 1 we’re stealing)
2) It defines our network boundary. Remember your powers of two – the far left bit we’re stealing is in the 2^7th spot – which is That means our networks are (if our 25th bit is a 0) and (if our 25th bit is a 1)
3) It defines our subnet mask. More on this later, but a /25 gives us a mask of
First 24 bits, locked
Last 8 bits.
We can play with these.

10. Subnetting
Remember, our possible network addresses are now (if our 25th bit is a 0) and (if our 25th bit is a 1):
Decimal:
Binary:
First 24 bits - locked
First 24 bits - locked
25th bit that we stole
To make 2 networks
25th bit that we stole
To make 2 networks
Available for Hosts
Available for Hosts

11. Subnetting Given: Network Definitions:
Decimal:
Binary:
Network Definitions:
Network Address Host Range Broadcast Address
Network
Network
First 24 bits - locked
First 24 bits - locked
25th bit that we stole
To make 2 networks
25th bit that we stole
To make 2 networks
Available for Hosts
Available for Hosts

12. A Step Further
Using our original network, /24, what do we do if we need 3 networks? Remember back on slide 9 how we “stole” 1 bit to give us two /25 networks (we stole 1, and 2^1 is 2 …so we have two networks)? We’re going to do the same thing. If we steal 2 bits, we have 2^2 available networks – so 4. We need 3, so this works and has the added benefit of giving us one more for growth.
Look back at slide 9 – we stole 1 bit, which could have a value of 1 or 0, making our networks or (because the value of the far left bit in any octet is 128), now we’re borrowing 2 bits, which looks like this:
First 24 bits - locked
25th and 26th bit that we stole
To make 2 networks
Available for Hosts

13. A Step Further
So, when we borrowed just 1 bit, the network boundaries were in 128 bit blocks (2^7). When we borrow 2 bits, the blocks are 64 (2^6). We have four networks, because each of our two borrowed bits can be either a 1 or 0 – this gives us the combinations of 00, 01, 10, and 11 – which makes the binary above look like one of these 4 combinations (with their decimal equivalents:
First 24 bits - locked
25th and 26th bit that we stole
To make 2 networks
Available for Hosts

14. A Step Further So, given this information from the last slide:
We have the networks:
Network Address Host Range Broadcast Network
Network
Network
Network

15. A Class B Example
Everything we’ve done so far as been with a class C address. They’re easier to learn on. Let’s take a look at this class be address and subnet the heck out of it:
/22
Whoa, wait up, what’s this /22 stuff you’re giving me? No worries, brah, I got you covered. First, convert to binary:
Let’s say we want to turn this into 4 networks. Easy enough by now, right? We just steal the last two bits of the 3rd octet. Work it out on paper. Those last two placeholders in the 3rd octet have a value of 2 and 1 (2^1, and 2^0). Remember, each bit can have a value of 1 or 0. So, our potential networks are:
Network Address Host Range Broadcast Network
Network
Network
Network
Note: With your new /24 networks, your subnet mask is now /22 was
First 22 bits - locked
Last 10 bits – we can play with

16. A Harder Class B Example
Your network assignment from your ISP was: /12
You need 7 networks. 2^3 is 8, so that works just fine. So we’re going to steal 3 bits to play with. Now, in binary, we have something that looks like this:
Let’s continue this on the next page…
First 12 bits - locked
Last 20 bits – we can play with
First 12 bits - locked
Last 17 bits for hosts
Next 3 bits, for subnetting

17. A Harder Class B Example
What are our networks in this example? The rightmost bit in the 3 bits that we’re subnetting has a value of 2 – in that second octet, that zero is in the 2^1 place – which makes our second network look like this (the first one is the original network we were given):
In decimal, this is
First 12 bits - locked
Last 17 bits for hosts
Next 3 bits, for subnetting

18. A Harder Class B Example
The other networks are , , , , , These are our 8 networks (again, remember the first one is still ). Here’s what that looks like
First 12 bits - locked
Last 17 bits for hosts
Next 3 bits, for subnetting

19. Summary and Tips
Subnetting is important. At very least you’ll need it to pass your CCENT/CCNA exam(s). If you ever setup a new network, office, or branch office in the real world, you’ll need it then as well.
Know your powers of 2 up to 2^7
Be able to convert from binary to decimal and back. You should know that is 224, and that 177 is
Practice. A lot. Consult the internet and search for subnetting practice. There’s tons of material out there that can keep you sharp with this stuff.
If you’re given an IP address and subnet mask, you should be able to quickly calculate the network ID. If you have a network ID and netmask, you should be able to list the broadcast address and range of usable IP addresses. It shouldn’t take you more than 10 seconds to do this.
You should practice to the point where you’re so sick of subnetting that by the time you walk into the exam, that you never want to see it again.