SUBNETTING SUPERNETTING AND VLSM
CCNA candidates need to be fluent in their understanding of IP addressing concepts. This
section describes how IP addresses are organized and analyzed.
This section would introduce subnetting. You will be able to subnet a network in your head after
going through this section. In addition, you will learn about Variable Length Subnet Masks
(VLSMs) and how to design a network using VLSMs. This would finish with summarization
techniques and configurations.
Variable Length Subnet Mask VLSM Tutorial
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Neither RIPv1 nor IGRP routing protocols have a field for subnet information, so the subnet
information gets dropped. What this means is that if a router running RIP has a subnet mask of a
certain value, it assumes that all interfaces within the classful address space have the same
subnet mask. This is called classful routing, and RIP and IGRP are both considered classful
routing protocols.
Classless routing protocols, however, do support the advertisement of subnet information.
Therefore, you can use VLSM with routing protocols such as RIPv2, EIGRP, and OSPF. The
benefit of this type of network is that you save a bunch of IP address space with it.
VLSM enables you to have more than one mask for a given class of address, albeit a class A, B,
or C network number.
VLSM, originally defined in RFC 1812, allows you to apply different subnet masks to the same
class address space Classful protocols, such as RIPv1 and IGRP, do not support VLSM. To
deploy VLSM requires a routing protocol that is classless—BGP, EIGRP, IS-IS, OSPF, or RIPv2,
for instance.
VLSM provides Two major advantages:
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more efficient use of addressing
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Ability to perform route summarization
when you perform classful subnetting, all subnets have the same number of hosts because they
all use the same subnet mask. This leads to inefficiencies. For example, if you borrow 4 bits on a
Class C network, you end up with 14 valid subnets of 14 valid hosts. A serial link to another router
only needs 2 hosts, but with classical subnetting, you end up wasting 12 of those hosts. Even
with the ability to use NAT and private addresses, where you should never run out of addresses
in a network design, you still want to ensure that the IP plan that you create is as efficient as
possible.
An efficient addressing scheme using VLSM.
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Find the largest segment in the area—the segment with the largest number of devices
connected to it.
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Find the appropriate subnet mask for the largest network segment.
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Write down your subnet numbers to fit your subnet mask.
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For your smaller segments, take one of these newly created subnets and apply a
different, more appropriate, subnet mask to it.
Write down your newly subnetted subnets.
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For even smaller segments, go back to step 4.
Route Summarization
Route summarization is the ability to take a bunch of contiguous network numbers in your
routing table and advertise these contiguous routes as a single summarized route.
Route summarization, or supernetting, is needed to reduce the number of routes that a router
advertises to its neighbor. Remember that for every route you advertise, the size of your update
grows. It has been said that if there were no route summarization, the Internet backbone would
have warped from the total size of its own routing tables back in 1997.
Routing updates, whether done with a distance vector or link-state protocol, grow with the number
of routes you need to advertise. In simple terms, a router that needs to advertise ten routes needs
ten specific lines in its update packet. The more routes you have to advertise, the bigger the
packet. The bigger the packet, the more bandwidth the update takes, reducing the bandwidth
available to transfer data. But with route summarization, you can advertise many routes with only
one line in an update packet. This reduces the size of the update, allowing you more bandwidth
for data transfer.
Summarization allows you to create a more efficient routing environment by providing the
following advantages:
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It reduces the size of routing tables, requiring less memory and processing.
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It reduces the size of updates, requiring less bandwidth.
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It contains network problems
Example of VLSM
Above image shows several branch offices using subnetted Class C (/26) addresses that provide
each branch with 62 possible host IPs. The branches are connected to the central office via pointto-point WAN links. The ideal mask to use for such a link is /30 because it provides only 2 hosts,
one for each end of the link. The problem arises when the routing protocols are configured: Prior
to VLSM, the /30 networks could not be used because the /26 networks existed in the same
system and the classful routing protocols could only advertise one mask per class of address. All
networks, including the little /30 links, had to use the same mask of /26. This wastes 60 IP
addresses on each WAN link.
With the implementation of VLSM-capable routing protocols, we can deploy a /30 mask on the
point-to-point links, and the routing protocols can advertise them as /30s along with the /26s in
the branches because the subnet mask for each network is included in the routing updates.
VLSM has allowed us to make the point-to-point link networks the ideal size (two hosts on each)
using /30 masks. This has allowed us to use a single subnetted Class C network for all the
addressing requirements in this scenario—and as you'll see, it makes a perfect opportunity to
summarize these routes. This is what is meant by "more efficient addressing"— in other words,
making networks the right size without depleting the limited address space or limiting future
growth.
Classless Interdomain Routing
Classless Interdomain Routing (CIDR), specified in RFC 2050, is an extension to VLSM and
route summarization.
With VLSM, you can summarize subnets back to the Class A, B, or C network boundary. For
example, if you have a Class C network 192.168.1.0/24 and subnet it with a 26-bit mask, you
have created four subnets. Using VLSM and summarization, you can summarize these four
subnets back to 192.168.1.0/24.
CIDR takes this one step further and allows you to summarize a block of contiguous class A, B,
and C network numbers. This practice is commonly referred to as supernetting. Today’s classless
protocols support supernetting. However, it is most commonly configured by ISPs on the Internet
using BGP.
Discontiguous subnets are not supported by classful protocols but are supported by classless
protocols. Classful protocols do not include the subnet mask when advertising network and
subnet numbers. When implementing route summarization, another thing you’ll need to consider
is that routing decisions, by a router, must be made on the entire destination IP address in the IP
packet header. The router always uses the longest matching prefix in the routing table.
CIDR allows you to summarize class networks together; VLSM allows you to summarize subnets
only back to the class network boundaryEach segment has a single network number and mask.
VLSM allows a class address, not a network segment, to have more than one subnet mask.
Method of Subnetting Benefits of Subnetting

Subnetting is the most tested topic of CCNA. In this article I would show you the method
of subnetting.
Benefit of Subnetting
Reduced network traffic
One network will not access the data of other network without the use of router. Thus we
can reduce the amount of data remain in one network. Less data less overhead, collision,
or broadcast storm.
Optimized network performance
This is a result of reduced network traffic.
Simplified management
It's easier to identify and isolate network problems in a group of Smaller connected
networks than within one gigantic network. Facilitated spanning of large geographical
distances Because WAN links are significantly slower and more expensive than LAN
links, a single large network that spans long distances can create problems in every area
earlier listed. Connecting multiple smaller networks makes the system more efficient.
Powers of 2
Powers of 2 are important to understand and memorize for use with IP subnetting.
21
2
29
512
22
4
210
1024
23
8
211
2048
24
16
212
4096
25
32
213
8192
26
64
214
16384
27
128
215
32768
28
256
216
65536
Before we go further let's get familiar with subnetting components
Subnet mask
A subnet mask is a 32-bit value that allows the receiver of IP packets to distinguish the
network ID portion of the IP address from the host ID portion of the IP address. Every IP
address is composed of a network component and a host component. The subnet mask
has a single purpose: to identify which part of an IP address is the network component
and which part is the host component. Subnet mask value 0 represent host ID while
subnet mask value 1 to 255 represents Network ID in ip address.
Classless Inter-Domain Routing (CIDR)
This slash notation is sometimes called CIDR (Classless Inter-Domain Routing) notation.
It's basically the method that ISPs (Internet service providers) use to allocate a number of
Addresses to a company, a home—a customer. The slash notation is simply the number
of 1s in a row in the subnet mask. The real reason to use CIDR notation is simply that it
is easier to say and especially to type.
Address Class and Default Mask
Subnetting happens when we extend the subnet mask past the default boundary for the
address we are working with. So it's obvious that we first need to be sure of what the
default mask is supposed to be for any given address. When faced with a subnetting
question, the first thing to do is decide what class the address belongs to. And later decide
what the default subnet mask is. One of the rules that Cisco devices follow is that a
subnet mask must be a contiguous string of 1s followed by a contiguous string of 0s.
There are no exceptions to this rule: A valid mask is always a string of 1s, followed by 0s
to fill up the rest of the 32 bits. (There is no such rule in the real world, but we will stick
to the Cisco rules here—it's a Cisco exam, after all.) Therefore, the only possible valid
values in any given octet of a subnet mask are 0, 128, 192, 224, 240, 248, 252, 254, and
255. Any other value is invalid.
Block Size
The process of subnetting creates several smaller classless subnets out of one larger
classful . The spacing between these subnets, or how many IP addresses apart they are, is
called the Block Size.
Network ID and Broadcast ID
The first address in a network number is called the network address, or wire number. This
address is used to uniquely identify one segment or broadcast domain from all the other
segments in the network.
The Broadcast ID
The last address in the network number is called the directed broadcast address and is
used to represent all hosts on this network segment. it is the common address of all hosts
on that Network ID. This should not be confused with a full IP broadcast to the address of
255.255.255.255, which hits every IP host that can hear it; the Broadcast ID hits only
hosts on a common subnet. A directed broadcast is similar to a local broadcast.
The main difference is that routers will not propagate local broadcasts between segments,
but they will, by default, propagate directed broadcasts.
Host Addresses
Any address between the network address and the directed broadcast address is called a
host address for the segment. You assign these middle addresses to host devices on the
segment, such as PCs, servers, routers, and switches.
Method of Subnetting
There are several method of subnetting. Different author different approach to calculate
the subnets. You should choose the method you can understand and perform subnetting
easily. Whatever approach you choose need conversion of decimal to binary. Cram up
this chart
27
26
25
24
23
22
21
20
128
64
32
16
8
4
2
1
To convert a decimal number into binary, you must turn on the bits (make them a 1) that
would add up to that number, as follows:
187 = 10111011 = 128+32+16+8+2+1
224 = 11100000 = 128+64+32
To convert a binary number into decimal, you must add the bits that have been turned on
(the 1s), as follows:
10101010 = 128+32+8+2 = 170
11110000 = 128+64+32+16 = 240
The IP address 138.101.114.250 is represented in binary as
10001010.01100101.01110010.11111010
The subnet mask of 255.255.255.224 is represented in binary as
11111111.11111111.11111111.11100000
Practical approach of subnetting
When faced with a subnetting question, the first thing to do is decide what class the
address belongs to. for examples:
192.168.1.1
The first octet is between 192 and 223 so it is a Class C address
Default mask for Class C: is 255.255.255.0
In exam default subnet mask is not subnetted. Now write down the given ip address as
shown here. Write down the default side of IP as it is and reset of part where actual
subnetting will perform in binary
192.168. 1 .00000001
255.255.255.00000000
(defaul maks)
Step 1:- calculate the CIDR value
CIDR are the on bit in subnet mask. As you can see in our example we have on bit only
in default side.
255.255.255.00000000
So our CIDR value is 24 + 0 = 24
Step 2:- calculate the Subnet mask
To calculate the subnet mask use the binary to decimal chart given above. Add the
decimal place value of on network bit.
<==H bit
255.255.255.00000000
N bit==>
In our example we are using on default mask so our subnet mask will be 255.255.255.0
Step 3:- calculate the Total Host
To calculate the total host count the H bit and use this formula
Total host = 2H
<==H bit
255.255.255.00000000
Total host = 28 = 256
Step 4:- calculate the Valid Host
Subtract 2 from Total host Every network or subnet has two reserved addresses that
cannot be assigned to a host. These addresses are called the Network ID and the
Broadcast ID, respectively. They are the first and last IPs in any network or subnet. We
lose those two IP addresses from the group of values that could be assigned to hosts.
Total host - 2
256 -2 = 254
Step 5:- calculate the Network
To calculate the Network count the N bit and use this formula
Network = 2N
255.255.255.00000000
N bit==>
Network = 20 = 1
Step 6:- Find out the block Size
Finding block size is very easy just subtract the subnet mask from 256
256 – Subnet mask
(only the last octal, don't include the default subnet mask)
256 - 0 = 256
Step 7:- Write down the subnet chart
Network 1
CIDR Value
/24
IP
Sunetmask
Net ID
192.168.1.0
255.255.255.0
First Valid
Host
192.168.1.1
255.255.255.0
Last Valid
Host
192.168.1.254
255.255.255.0
Broadcast ID
192.168.1.255
255.255.255.0
Subnetting of CIDR /25
Now do the subnetting of CIDR /25 using same method
Step 1:- calculate the CIDR value CIDR = sum of all on bit in subnet mask
255.255.255.10000000
So our CIDR value is 24 + 1 = 25
Step 2:- calculate the Subnet mask
Add the decimal place value of on network bit.
<==H bit
255.255.255.10000000
N bit==>
In our example we have one on bit and as you can see in decimal chart the place value of
1000000 is 128 so our subnet mask will be 255.255.255.128
Step 3:- calculate the Total Host
Total host = 2H <==H bit 255.255.255.10000000 Total host = 27 = 128
Step 4:- calculate the Valid Host
Subtract 2 from Total host
Total host - 2
128 -2 = 126
Step 5:- calculate the Network
To calculate the Network count the N bit and use this formula
Network = 21 255.255.255.10000000 N bit==> Network = 21 = 2
Step 6:- Find out the block Size
256 – Subnet mask (only the last octal, don't include the default
subnet mask) 256 - 128 = 128
With help of block size you can easy find out the network ID and broadcast ID of all
possible networks as we have 8 bits in one octal those can give maximum of 28 = 256
decimal number
We start from 0 so it will end up on 255 (Do not get confuse because we are counting
from 0 not from 1 so the last digit will be 255 not 256. It will 256 only when you count
from 1 ). All subnetting will perform between these two numbers.
Create a table of x Columns where x is the number of your network
First ip of first network will always be 0 and last ip of last network will be 255 fill its in
chart
Now you have network ID of first network and broadcast ID of last network.
Now add block size in the first ip of first network to get the network ID of second
network and so on till we get the network id of last network
First network ID 0 Second Network ID 0 +128 = 128
Fill this in Chart.
As you can see from 128 next network is started so the last IP of first network will be 127
fill it in chart. With this method you can fill the last ip of all networks.
Now you have first ip ( network ID ) of all networks and the last ip (Broadcast ID) of all
networks. At this point you can easily fill the valid ip in each network. As valid hosts are
all ip address those fall between network ip and host ip.
Step 7:- Write down the subnet chart
CIDR /25
Network 1
Network 2
Net ID
192.168.1.0
192.168.1.128
First Valid
Host
192.168.1.1
192.168.1.129
Last Valid
Host
192.168.1.126
192.168.1.254
Broadcast ID
192.168.1.127
192.168.1.255
Binary ANDing
Binary ANDing is the process of performing multiplication to two binary numbers. In the
decimal numbering system, ANDing is addition: 2 and 3 equals 5. In decimal, there are
an countless number of answers when ANDing two numbers together. However, in the
binary numbering system, the AND function give up only two possible outcomes, based
on four different combinations. These answers, can be displayed as a truth table:
0 and 0 = 0 1 and 0 = 0 0 and 1 = 0 1 and 1 = 1
You use ANDing most often when comparing an IP address to its subnet mask. The end
result of ANDing these two numbers together is to give up the network number of that
address.
Example Question
What is the network number of the IP address 192.168.100.115 if it has a subnet mask of
255.255.255.240?
Answer
Step 1 Convert both the IP address and the subnet mask to binary:
192.168.100.115 = 11000000.10101000.01100100.01110011
255.255.255.240 = 11111111.11111111.11111111.11110000
Step 2 Perform the AND operation to each pair of bits—1 bit from the address ANDed to
the corresponding bit in the subnet mask. Refer to the truth table for the possible
outcomes:
192.168.100.115 = 11000000.10101000.01100100.01110011
255.255.255.240 = 11111111.11111111.11111111.11110000
ANDed result = 11000000.10101000.01100100.01110000
Step 3 Convert the answer back into decimal:
11000000.10101000.01100100.01110000 = 192.168.100.112
The IP address 192.168.100.115 belongs to the 192.168.100.112 network when a mask of
255.255.255.240 is used.
My easy method
Conversion of decimal to binary and vice versa to get network ID is too time consuming
process in exam. So I found this easy method.
Step 1:- Decide from which class this IP belongs and what's its default subnet mask
As given IP have 192 in its first octal so it's a class C IP. And default subnet mask of
class C is 255.255.255.0
Step2:- Find out the block size. ( As we describe above)
256 -240 = 16
Step3:- Write down all possible network using block size till we do not get our host
partition in middle of two network
0,16,32,48,64,80,96,112,128,
As our host number is 115 which fall in the network of 112 so our network ID is
192.168.1.112
And our host's broad cast ID is 192.168.1.127 as from 128 onward next network will
start. Easy as I promise
IP Subnet Practice tools
Click the [New Problem] button to start
Given the IP address
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Enter the information in the IP address field below. [Check] if your answer is right or
[Show] the answer
IP address
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Answer
OK
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Last Host
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