Welcome to the subnetting.net subnetting tutorial. This training session will teach
you everything you need to know to pass any exam that requires TCP/IP knowledge.
In addition, the methods you will learn here will allow you to not only solve subnetting
questions, it will help you increase your speed and master TCP/IP concepts.
For many entry-level to intermediate computer networking exams, the need to quickly
solve subnet based questions is apparent. In addition, the mastery of these concepts
will give you a major advantage when you sit for the exam. For example, some have
estimated that the CCNA exam has around 40% subnetting questions. When you are done
with this tutorial, these questions will be no harder than simple addition or subtraction.
To put this is real life terms, suppose the CCNA exam has 50 questions and you are given
around 20 subnetting questions. When you learn to subnet, you should expect to ace
these 20 questions. In other words, you would get 100% on 40% of the exam. This makes
the remainder of your study time much more productive.
TCP/IP Subnetting Very Basic Watch a Video on this Topic
The first thing we need to look at is - What is an IP address? How does it work?
Here is kind of a loose analogy that should make this very clear. Think of an IP
address as your postal address. Let's take 191.54.38.15 for example. In our analogy,
each set of numbers may represent a geographic area. As we move from the left to
the right, the geographic area continuously becomes more and more specific.
191 = Your State
54 = Your Town
38 = Your Apartment building
15 = Your Apartment Number
To have TCP/IP, at some point, you must have a router. It doesn't make much difference
if you have a router in your house, or a router in your business. They basically
provide the same functionality. To further this analogy, think of a router as a
post office. If you were going to send a letter (or in this case email) to another
IP address, your router will decide what to do with that message.
If you put the outgoing letter by your mail box, a few things would happen based on the destination postal address (IP Address).
If the letter was being mailed to someone in your own building, the router or postal carrier (switch) would know that it does not have to leave the building (your subnet) and would send it directly.
Intra-Subnet = The postal carrier (switch), would know that the destination address is local and would send it directly to the recipient.
For example, if you were on IP 191.54.38.15 and you mailed your letter to 191.54.38.16, the letter would never leave your building.
The concept of a subnet mask, is what tells the mail carrier which addresses are within a given network.
As our analogy stands right now, we don't know how many apartments are in your building. We will get into the subnet masks later, for now let's assume there are 30 apartments in your building and the building next-door also has 30 apartments.
To mail a letter within your own apartment building (subnet), a router/post office is not required.
Okay, easy enough. What happens if you want to send it to the building next-door?
Your mail carrier would take the letter to the post office in your town, and the
post office would route the letter to the building next-door. This is what is called
a "hop". So if your address was 191.54.38.15 and
you wanted to send your letter to 191.54.39.15,
your local router would simply forward your request to the Post Office that knows
how to get to 191.54.39.15.
Now, suppose your letter was addressed to a different town in your state, let's say 191.55.1.10.
The process is basically the same, but it will include more "hops".
In this scenario, your mail carrier would take the letter to your local post office, it would then be sent to a hub within your state, then the destination town's post office, then finally to the end recipient.
The communication process would look something like this:
- Your Town Post Office (ISP): I don't know anything about 191.55.x.x, I only know 191.54.x.x. Therefore, I'm going to send it to the "hub" post office (in IP terms this is basically what a default route does).
- Hub Post Office: This letter is addressed to 191.54.x.x, those addresses are serviced by the post office in the destination town.
- Destination Town: Ok, this letter is addressed to 191.54.x.x, I know where that is so I'm going to send this letter directly to the end recipient.
Of course, this is only a loose analogy and the IP addresses used to describe the process could have been anything.
The takeaway from this first lesson is the router in your home or business, simply does not know where every possible destination IP address is located.
TCP/IP Address Construct Watch a Video on this Topic
A TCP/IP address is a 32 bit, binary number that has been converted to decimal.
Whenever the word binary comes up, many people immediately become concerned. No
worries, this is going to be very easy. Here are a few simple definitions to help
the process:
Octet = An octet is simply a group of 8 numbers. The IP address 191.168.1.1 has
four octets: 191, 168, 1, and 1. I know I said there are 8 numbers and I only showed
you four, but keep in mind that IP is in binary, not decimal. Each set of numbers
represent 8 binary digits (zeroes or ones).
Bits = A bit is simply a single character. In other words, 32 bits equals 32 zeroes
or ones.
Binary = A number system that is based on two characters - 0 & 1.
Okay, now let's convert our decimal IP address of 191.54.38.15 into binary:
Remember that each octet represents eight binary digits - Each placement of those
binary digits represents a decimal number. To start, let's look at a simple three
digit binary number:
Our decimal numbers on the bottom always start with 1 and double for each placement
going from right-to-left. To convert to a decimal number, we will add a 1 above
every decimal number we want to add together to get to our decimal number and a
zero for every number we are not going to add.
|
Decimal Number 3
|
|
0
|
1
|
1
|
|
----
|
----
|
----
|
|
4
|
2
|
1
|
Now we simply add each decimal number (on the bottom) each time we see a binary
1. Thus, decimal 3 = 011
|
Decimal 4 = Binary 100
|
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1
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0
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0
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|
----
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----
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----
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4
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2
|
1
|
|
Decimal 5 = Binary 101
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1
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0
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1
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----
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----
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----
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4
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2
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1
|
|
Decimal 191 = Binary 10111111
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1
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0
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1
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1
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1
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1
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1
|
1
|
|
----
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----
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----
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----
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----
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----
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----
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----
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128
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64
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32
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16
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8
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4
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2
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1
|
|
Decimal 54 = Binary 00110110
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0
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0
|
1
|
1
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0
|
1
|
1
|
0
|
|
----
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----
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----
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----
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----
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----
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----
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----
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128
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64
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32
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16
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8
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4
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2
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1
|
|
Decimal 38 = Binary 00100110
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0
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0
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1
|
0
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0
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1
|
1
|
0
|
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----
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----
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----
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----
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----
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----
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----
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----
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128
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64
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32
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16
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8
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4
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2
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1
|
|
Decimal 15 = Binary 00001111
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0
|
0
|
0
|
0
|
1
|
1
|
1
|
1
|
|
----
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----
|
----
|
----
|
----
|
----
|
----
|
----
|
|
128
|
64
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32
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16
|
8
|
4
|
2
|
1
|
|
191
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54
|
38
|
15
|
|
10111111
|
00110110
|
00100110
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00001111
|
191.54.38.15 = 10111111.00110110.00100110.00001111
Subnet Mask
A subnet mask is used to determine where the network ends and the hosts begin. We
get this information by converting the decimal numbers within the subnet mask to
binary. The 1's in our binary subnet mask represents the network portion of our
IP address, and the 0's represent the hosts. Here's an example:
IP Address: 192.168.1.0
Subnet Mask: 255.255.255.0
The first thing we do is convert our first octet to binary:
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
|
----
|
----
|
----
|
----
|
----
|
----
|
----
|
----
|
|
128
|
64
|
32
|
16
|
8
|
4
|
2
|
1
|
When we add up all the decimal numbers underneath our binary 1's, we get 255 (128+64+32+16+8+4+2+1).
In this example, the next two octets are also all 1's. And as you probably have
guessed the fourth and final octet in our example is all 0's or all hosts.
So in binary, 255.255.255.0 is equal to:
11111111.11111111.11111111.00000000
Wherever our 1's stop and our zeros begin is called our network boundary:
11111111.11111111.11111111.|00000000
In this example, the first three octets are our network and the last one represents
our hosts:
Subnet = 192.168.1.0
Hosts* = 192.168.1.1 - 192.168.1.254 (E.g. 192.168.1.1,
192.168.1.2, 192.168.1.3,
etc...)
*The number of hosts on any network is determined by taking 2 to the power of the number of binary
zero's. In this example, 8 0's = 2^8 = 256. However, we have
to remove the first and last address from the pool of usable addresses. We will
discuss this later, for now suffice it to say that the actual formula to determine
how many hosts is (2^n)-2, where n = the number of binary zero's.
In our example n=8, so we have 2^8-2=256-2=254. We have a single network with 254 usable IP addresses for hosts (PC's,
printers, etc...).
Now, let's take a look at what happens if we change our subnet mask:
IP Address: 192.168.1.0
Subnet Mask: 255.255.0.0
Just like the previous example, the first thing we need to do is to determine our network
boundary by converting our decimal subnet mask to binary:
11111111.11111111.|00000000.00000000
Subnet = 192.168.0.0
Hosts = 192.168.0.1-192.168.255.254
Note that we can't use 192.168.0.0 as a host because it is the network address and 192.168.255.255 also can't be used because it is reserved for the broadcast address.
However, all addresses in between the network address and the broadcast address can be assigned to hosts.
Although it may look strange, addresses such as 192.168.0.255, 192.168.1.0, and 192.168.255.0 are all valid host addresses in the 192.168.0.0/16 network.
So now we can use the same formula to determine the number of assignable hosts -
16 zeros = 2^16-2 = 65,534 hosts.
IP Address Classes
There are five classes of IP addresses:
|
Class
|
Purpose
|
First Octet Range
|
Maximum Hosts
|
Total Networks
|
|
Class A
|
Very Large Networks
|
1-126*
|
16,777,216 (2^24)
|
128 (2^7)
|
|
Class B
|
Large Enterprise
|
128-191
|
65,536 (2^16)
|
16,384 (2^14)
|
|
Class C
|
Small Business
|
192-223
|
256 (2^8)
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2,097,152 (2^21)
|
|
Class D
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Multicast
|
224-239
|
N/A
|
N/A
|
|
Class E
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Experimental
|
240-255
|
N/A
|
N/A
|
*IP addresses that begin with 0 or 127 are reserved for various functions.
Some find it easier to remember these addresses via the first binary bits of each
network type:
0 = Class A
10 = Class B
110 = Class C
1110 - Class D
1111 - Class E
Private Network Ranges
Each class of IP addresses has a private address range.
These ranges were created to conserve public IP addresses.
Instead of issuing a real IP address to every host on your network, you can issue a private address and translate the private address to a public one when you communicate with the Internet.
For example, a network with 200 or even 200,000 hosts, could use just one public IP address.
This process is called NAT/PAT and will be covered in another lesson.
Class A Private Address = 10.0.0.0/8. If an IP address begins with the number 10, it is a class A private address.
Class B Private Address = 172.16.0.0/12. If an IP address begins with 172.16-31, it is a class B private address.
Class C Private Address = 192.168.0.0/16. If an IP address begins with 192.168, it is a class C private address.
Chances are if you look at the IP address of your PC right now, you will have a private IP address in one of the above ranges.
If you are at home, your router is likely running DHCP and issued your PC a private IP Address.
When you communicate on the Internet, the public IP address that has been issued by your ISP will be used.
The router in your home is translating your private address to your public address.
Key Topic: A private IP address cannot be routed over the Internet, but can be routed throughout your internal network.
Subnet Mask Short-Hand
Now that we have basic subnetting figured out and know how to convert to binary,
this will be a very brief section on subnet mask shorthand.
Instead of writing out a full subnet mask such as 255.255.255.0, we can shorten
this to /24. To come to this number, we simply add the number of subnet bits (1's)
in our subnet mask.
Subnet Mask Shorthand to Decimal Video Example:
Subnet Mask Chart
|
Decimal
|
Shorthand
|
Binary
|
|
255.0.0.0
|
/8
|
11111111.00000000.00000000.00000000
|
|
255.128.0.0
|
/9
|
11111111.10000000.00000000.00000000
|
|
255.192.0.0
|
/10
|
11111111.11000000.00000000.00000000
|
|
255.224.0.0
|
/11
|
11111111.11100000.00000000.00000000
|
|
255.240.0.0
|
/12
|
11111111.11110000.00000000.00000000
|
|
255.248.0.0
|
/13
|
11111111.11111000.00000000.00000000
|
|
255.252.0.0
|
/14
|
11111111.11111100.00000000.00000000
|
|
255.254.0.0
|
/15
|
11111111.11111110.00000000.00000000
|
|
255.255.0.0
|
/16
|
11111111.11111111.00000000.00000000
|
|
255.255.128.0
|
/17
|
11111111.11111111.10000000.00000000
|
|
255.255.192.0
|
/18
|
11111111.11111111.11000000.00000000
|
|
255.255.224.0
|
/19
|
11111111.11111111.11100000.00000000
|
|
255.255.240.0
|
/20
|
11111111.11111111.11110000.00000000
|
|
255.255.248.0
|
/21
|
11111111.11111111.11111000.00000000
|
|
255.255.252.0
|
/22
|
11111111.11111111.11111100.00000000
|
|
255.255.254.0
|
/23
|
11111111.11111111.11111110.00000000
|
|
255.255.255.0
|
/24
|
11111111.11111111.11111111.00000000
|
|
255.255.255.128
|
/25
|
11111111.11111111.11111111.10000000
|
|
255.255.255.192
|
/26
|
11111111.11111111.11111111.11000000
|
|
255.255.255.224
|
/27
|
11111111.11111111.11111111.11100000
|
|
255.255.255.240
|
/28
|
11111111.11111111.11111111.11110000
|
|
255.255.255.248
|
/29
|
11111111.11111111.11111111.11111000
|
|
255.255.255.252
|
/30
|
11111111.11111111.11111111.11111100
|
There is no need to memorize these, as you practice they will become second nature.
Reserved Host Addresses
As mentioned earlier, for every subnet, the first and the last IP addresses are reserved:
First IP Address = Network ID
Last IP Address = Broadcast Address. The broadcast address is used for a specific
type of network traffic that is destined for every device attached to a network.
Example:
IP Address = 192.168.1.1
Subnet Mask = 255.255.255.0
Network ID = 192.168.1.0
Broadcast Address = 192.168.1.255
Usable IP's = 192.168.1.1 - 192.168.1.254
Subnet 0
In the not too distant past, it was considered bad practice to use the first network
in your subnet range. For example, if we had the IP range of 192.168.1.1 255.255.255.192,
our networks would be:
192.168.1.0, 192.168.1.64, 192.168.1.128, 192.168.1.192
In this example, the first network (192.168.1.0) would not be used. However, this
command in Cisco equipment is really never used and the default Cisco settings assume
subnet 0 will always be used. If you take a Cisco exam and they indicate that the
subnet zero command is on, you have to remove the first network from your calculations.
However, chances are this will not happen on any current exam. You should always
assume the first network can be used unless you are told otherwise.
Subnetting Cheat Sheet
Now that we have all the basics down, we are going to start the actual lesson on subnetting.
The first thing we are going to do is create a Subnet Cheat Sheet.
This will help you solve subnetting questions very quickly and without errors.
I highly recommend you follow along and create your own subnet cheat sheet.
In addition, this is the first thing you should write down on your scratch pad when you sit for the actual CCNA exam.
Do this before you even look at the first question!
Video Lesson:
Written Lesson:
The first step is to create a grid with three columns and 14 rows:
| Bits | Block Size / Exponents | Decimal Mask Value |
|---|
| 0 | 1 | 255 |
| 1 | 2 | 254 |
| 2 | 4 | 252 |
| 3 | 8 | 248 |
| 4 | 16 | 240 |
| 5 | 32 | 224 |
| 6 | 64 | 192 |
| 7 | 128 | 128 |
| 8 | 256 | |
| 9 | 512 | |
| 10 | 1024 | |
| 11 | 2048 | |
| 12 | 4096 | |
Label the columns: Bits, Block Size/Exponent, and Decimal Mask Value:
| Bits | Block Size / Exponents | Decimal Mask Value |
|---|
| 0 | 1 | 255 |
| 1 | 2 | 254 |
| 2 | 4 | 252 |
| 3 | 8 | 248 |
| 4 | 16 | 240 |
| 5 | 32 | 224 |
| 6 | 64 | 192 |
| 7 | 128 | 128 |
| 8 | 256 | |
| 9 | 512 | |
| 10 | 1024 | |
| 11 | 2048 | |
| 12 | 4096 | |
For the bits column, begin at the top with a zero and count all the way to 12:
| Bits | Block Size / Exponents | Decimal Mask Value |
|---|
| 0 | | |
| 1 | | |
| 2 | | |
| 3 | | |
| 4 | | |
| 5 | | |
| 6 | | |
| 7 | | |
| 8 | | |
| 9 | | |
| 10 | | |
| 11 | | |
| 12 | | |
For the Block Size / Exponent column, start at the top with a 1, and double the preceding number all the way to the end:
| Bits | Block Size / Exponents | Decimal Mask Value |
|---|
| 0 | 1 | |
| 1 | 2 | |
| 2 | 4 | |
| 3 | 8 | |
| 4 | 16 | |
| 5 | 32 | |
| 6 | 64 | |
| 7 | 128 | |
| 8 | 256 | |
| 9 | 512 | |
| 10 | 1024 | |
| 11 | 2048 | |
| 12 | 4096 | |
In the decimal value column, start at the row that corresponds to the block size / exponent of 128, and enter the value 128.
For each row going up, add the value from the row above in the Block Size / Exponent column.
Your final sheet should look like this:
| Bits | Block Size / Exponents | Decimal Mask Value |
|---|
| 0 | 1 | 255 |
| 1 | 2 | 254 |
| 2 | 4 | 252 |
| 3 | 8 | 248 |
| 4 | 16 | 240 |
| 5 | 32 | 224 |
| 6 | 64 | 192 |
| 7 | 128 | 128 |
| 8 | 256 | |
| 9 | 512 |
| 10 | 1024 |
| 11 | 2048 |
| 12 | 4096 |
The rows corresponding to the bit values 0-7 represent one IP octet.
The remaining rows are a convenience to quickly determine various exponents. E.g. 2^11 = 2048 or 2^10 = 1024.
Putting it all Together
For this next section, I suggest you create your own cheat sheet so you do not have
to scroll up to follow along. There are basically three types of subnetting questions
that will be asked in our practice section, on any exam, or you need to answer in
real life. We will go over these question types and show you how to use the cheat
sheet to find the answer very quickly.
Question Type 1 Video Example:
Question Type 1 Written Example: What is the Network ID, Broadcast Address, First Usable
IP, or Last Usable IP on the subnetwork that the node 192.168.1.15/26 belongs to?
No matter which of the four criteria the question asks, you should always follow
these identical steps. This makes solving the actual question very easy.
Step 1. Convert the shorthand subnet mask to decimal.
- /26 = 255.255.255. + Two additional subnet bits.
- Go to your cheat sheet, start at the bottom (128) and count up two, starting with
128. You should get to 192.
- Thus, our decimal subnet mask is 255.255.255.192
Step 2. Determine the block size.
- The block size is listed in the block size column parallel to your decimal mask.
- The block size is 64.
Step 3. What is my Network ID?
- Since we are working in the fourth octet and the block size is 64, the first network
is 192.168.1.0.
Step 4. What is the next Network ID?
- Again, we look at our block size of 64 to determine the next network is 192.168.1.64.
- Network ID (First IP in the subnet): 192.168.1.0
- Broadcast address (last IP in the subnet): 192.168.1.63
- First Usable IP (the address after the network ID): 192.168.1.1
- Last Usable IP (the address before the broadcast address): 192.168.1.62
Notes: The block size determines the networks. Since the block size is 64, our networks
are 192.168.1.0, 192.168.1.64, 192.168.1.128, and 192.168.1.192. To determine your
network, you just have to find the range that includes your IP address. In this
example, it is the first network. If your IP was 192.168.1.72, you would belong
to the second network 192.168.1.64 - 192.168.1.127.
Let's quickly go through one more: Enter the last valid host on the network that the host 172.30.118.230/23 is a part of:
Step 1. Convert the shorthand subnet mask to decimal.
- /23 = 255.255. + Seven additional subnet bits.
- Go to your cheat sheet, start at the bottom (128) and count up seven, starting with 128. You should get to 254.
- Thus, our decimal subnet mask is 255.255.254.0.
Step 2. Determine the block size.
- The block size is listed in the block size column parallel to your decimal mask.
- The block size is 2.
Step 3. What is my Network ID?
- Since we are working in the third octet and the block size is 2, the first network is 172.30.118.0. (2 * 59 = 118).
We are simply looking for the largest multiple of the block size without going over the given octet number (118).
Step 4. What is the next Network ID?
- Again, we look at our block size of 2 to determine the next network is 172.30.120.0 (118 + 2).
- Network ID (First IP in the subnet): 172.30.118.0
- Broadcast address (last IP in the subnet): 172.30.119.255
- First Usable IP (the address after the network ID): 172.30.118.1
- Last Usable IP (the address before the broadcast address): 172.30.119.254 (This is the answer to the specific question that was asked.)
Question Type 2 Video Example:
Question Type 2 Written Example: How many subnets and hosts per subnet can you get from the
network 192.168.1.0 255.255.255.224?
Step 1: Determine the classful mask.
- 192 = Class C
- Class C default mask = 255.255.255.0
Step 2: Determine how many additional subnet bits exist beyond the classful boundary.
- Since this is a class C address, we are only going to look at the fourth octet.
Since the decimal mask is 224, we know there were three subnet bits added (counting
up from the bottom of our cheat sheet).
Step 3: Determine how many host bits remain.
- We are still only working in the fourth octet. Since three bits went to the subnet
in step 2, we have five host bits (0's) remaining.
Step 4: Find the exponents of both subnet bits and host bits using the cheat sheet:
- Subnet Bits = 2^3 = 8
- Host Bits = 2^5-2 = 30
The answer is 8 subnets and 30 hosts per subnet.
Question Type 3 Video Example:
Question Type 3 Written Example: You have been asked to create a subnet mask for the 172.16.0.0
network. Your organization requires 900 subnets, with at least 50 hosts per subnet.
What subnet mask should you use?
Step 1: Determine how many subnet bits (1's), you have to add to the classful boundary
to cover the number of required subnets.
- The IP address given was a class B address, making the first 16 subnet bits static.
- Using the cheat sheet, find the exponent of 2 that is equal to or greater than the
number of subnets we require (900). We can quickly see that 10 additional subnet
bits will give us 1,024 subnets. Make note of the corresponding subnet mask. In
this case, 255.255.255.192. The third octet is eight 1's, and the four is two 1's.
We can count up from the bottom on our cheat sheet to get to 192.
Step 2: Confirm the number of remaining 0's will cover our required hosts. In this
case, there are 6 remaining 0's - 2^6-2=62, which is more than enough for our host
requirements.
Our subnet mask is 255.255.255.192. Giving us 1024 subnets and 62 hosts per subnet.
For more examples, visit our Tips & Tricks page.
Advanced Topics
Variable Length Subnet Mask (VLSM)
You are a network administrator and you have been assigned the task of setting up
the IP network. You have a single class C IP address range and have to setup the
following networks:
- Network A = 100 Hosts
- Network B = 40 Hosts
- Network C = 20 hosts
- Network D = 10 hosts
- Network E = 6 Hosts
- Network F = 2 Hosts
|
IP Range
|
Network
|
Num IPs
|
Subnet Mask
|
|
0-127
|
Network A
|
128
|
/25
|
|
128-191
|
Network B
|
64
|
/26
|
|
192-223
|
Network C
|
32
|
/27
|
|
224-239
|
Network D
|
16
|
/28
|
|
240-247
|
Network E
|
8
|
/29
|
|
248-251
|
Network F
|
4
|
/30
|
It's really that easy:
- Add up all host requirements to make sure you have enough in a single block.
- Work from the top down (largest host requirements) removing the number of required hosts for each network.
Route Summarization
A summarized route can increase performance of a router by limiting the route table and reducing routing traffic. Consider the following question:
You have a router with four different LANs attached:
- Network 1 = 192.168.0.0/24
- Network 2 = 192.168.1.0/24
- Network 3 = 192.168.2.0/24
- Network 4 = 192.168.3.0/24
How would you summarize these networks with a single statement?
Step 1: Convert all four IP addresses to binary and align them in a list:
| Decimal |
Binary |
| 192.168.0.0 | 11000000.10101000.00000000.00000000 |
| 192.168.1.0 | 11000000.10101000.00000001.00000000 |
| 192.168.2.0 | 11000000.10101000.00000010.00000000 |
| 192.168.3.0 | 11000000.10101000.00000011.00000000 |
Step 2: In the binary section of the grid, count the common bits from left to right.
In this example, the first two octets are identical, so we know the first 16 bits are common across all four networks.
In addition, the first six bits of the third octet are also common.
It may be easier to draw a line through all four binary numbers at the point where the bit pattern changes.
Step 3: Count the number of common bits in the mask.
In this example, the first 22 bits are common across all four networks.
The answer is 192.168.0.0/22.
Let's go through one more:
- Network 1 = 10.10.0.0/16
- Network 2 = 10.11.0.0/16
- Network 3 = 10.12.0.0/16
- Network 4 = 10.13.0.0/16
- Network 5 = 10.14.0.0/16
- Network 6 = 10.15.0.0/16
Here is the list in binary:
| Decimal |
Binary |
| 10.10.0.0 | 00001010.00001010.00000000.00000000 |
| 10.11.0.0 | 00001010.00001011.00000000.00000000 |
| 10.12.0.0 | 00001010.00001100.00000000.00000000 |
| 10.13.0.0 | 00001010.00001101.00000000.00000000 |
| 10.14.0.0 | 00001010.00001110.00000000.00000000 |
| 10.15.0.0 | 00001010.00001111.00000000.00000000 |
In this example, the first octet is identical and so are the first five bits of the second octet. 8+5=13.
The answer is 10.8.0.0/13.
The 8 is derived from our block size of 8 (refer to your cheat sheet).
Now it's time to Practice Subnetting!