easy ip addressing and subnetting manual for starters

2013

CDi Communications, Inc.

Netwind Learning Center, 4327 South Hwy 27, Suite 331 Clermont

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Copyright © 1996‐2013 Netwind Learning Center / CDi Communications, Inc.

Easy IP Addressing and Subnetting Manual for Starters

1

Table of Contents What Is An IP Address? .............................................................................................................................................. 2

IP Addresses (Binary to Decimal and Decimal to Binary Conversion ) ...................................................................... 3

Public and Private IP Addresses................................................................................................................................. 5

Classes of IP Addresses .............................................................................................................................................. 5

Class A IP Addresses: ............................................................................................................................................. 6

Class B IP Addresses: ............................................................................................................................................. 6

Class C IP Addresses: ............................................................................................................................................. 7

Subnet Mask: ......................................................................................................................................................... 8

Sub netting: ........................................................................................................................................................... 8

Sub Netting a Class C IP Address ............................................................................................................................... 9

Sub netting a Class B Address: ................................................................................................................................. 11

Sub netting Class A Address: ................................................................................................................................... 12

Sub netting Exercise ........................................................................................................................................... 14

VLSM (Variable Length Subnet Masking): ............................................................................................................... 15

VLSM Practical Example ...................................................................................................................................... 16

Super netting or Route Summarization .................................................................................................................. 18

2

What Is An IP Address? In the real life, we as human beings, trace each other via the use of different sort of addresses and location

services. The same pattern was applied when computer networks were designed, in the form of IP addressing.

An IP address is just like the home address of a computer node! As is the rule in real life, when we want to send

some Mail, we write a destination address on it and it is delivered by the postal services to the concerned

person. Same is the case in computer networks, when one Computer wants to send some data to another

computer, it writes down the destination address on the data ( packet in computer networks) and the packet is

sent via the Postal service ( our network services) of the computer system.

In simple words, an IP Address is a decimal representation of the address of different network nodes which

enable them to exchange data packets with each other and hence many network applications. So what is the

abbreviation of IP? Internet Protocol, so simple!

The IP address evolution began in 1969. The original IP address was of 5 bits only! Which means according to

binary calculations it was able to cover a network of only 32 nodes! ( 2 to the power of 5 = 32), which was

enough at that time for the experimental requirements of that time, mostly interconnection of different

research organizations. Gradually it was increased to 32 bits, the currently used range in IPv4, which is enough

for around 4 billion network nodes only! (Only? Yes, because it has become short for the ever expanding human

world, that’s why techno geek has moved toward IPv6). Especially advent of smart phones and smart sensor

devices which are able to connect to internet through easily available wifi spots and 3 G cellular connection will

make it possible in the near future that a tech savvy person will be carrying around 4 ‐10 devices with him/her

with a public IP address.

In technical terms, IPv4 is represented by 4 blocks, each separated by a dot (.) and each block composed of 8

bits, represented as follows:

00000000.00000000.00000000.00000000 10000000.00000000.00000000.00000000 11000000.00000000.00000000.00000000 Don’t give up if you are learning for the first time, as IP addresses are not represented in binary, as it would not

be able for everyone to remember the binary digits, for ease they are represented in decimal representation of

its binary form.

So an IP address: 192.168.100.2 and 11000000.10101000.01100100.00000010 are same.

In simple words, each block can be written as:

11000000 = 192 10101000 =168 01100100 =100 00000010 =2 As now we have discussed IP addressing, its representation/bits requirements, and now we will do a little

discussion on how to convert from Binary to decimal and decimal to Binary.

3

IP Addresses (Binary to Decimal and Decimal to Binary Conversion )

Now we will discuss how to convert a binary representation of an IP address in decimal one and vice versa. We

will take following sample IP Address:

11000000.10101000.01100100.00000010

Each block is comprised of 0 or 1, 0/1 in binary represent On/Off states respectively. We will take below chart to

convert the above binary into decimal or base 10 systems. To convert the first Octet (an octet is composed of 8

bits) into decimal:

11000000 = 1*128 + 1*64 + 0*32 + 0*16 + 0*8 + 0*4 + 0*2 + 0*1 = 128 + 64 + 0 + 0 + 0 + 0 + 0 + 0 = 192

10101000 = 1 *128 + 0*64 + 1*32 + 0*16 + 1*8 +0*4 + 0*2 + 0*1 = 128 + 0 + 32 + 0 + 8 + 0 + 0 + 0 = 168

And so on

In the above conversion process each bit in (11000000) is multiplied by its corresponding bit position value in

decimal starting from least significant bit to the most significant bit. Please remember below mentioned chart

for efficient conversion of Binary into decimal:

The 8th bit position will be multiplied by 128, 7th bit position will be multiplied by 64 and so on!

Conversion from Decimal to Binary is a little tricky. Suppose we want to convert 15 from decimal into binary.

Consider below mentioned chart, which combination of digits added together can give a sum of 15? After a little

brainstorming on below mentioned chart we conclude 8+4+2+1 sums up to 15, so we will change the status of

these bits to ON (1) and will turn OFF (0) all the remaining bits:

128 64 32 16 8 4 2 1

128 64 32 16 8 4 2 1

0 0 0 0 1 1 1 1

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So the resulting value of 15 in an 8 bit binary representation is 00001111! Another example to solidify the

concepts:

Conversion of 130 into binary:

130 can be made from summing 128 and 2, so we will ON these bits and will turn OFF the remaining bits:

128 64 32 16 8 4 2 1

1 0 0 0 0 0 1 0

So 130 = 10000010 in binary, I hope now you can easily convert between binary and decimals. The interesting

thing about the above chart is that, it can be used for binary to decimal conversion as well. Suppose we want to

convert 11100100 into decimal, simply put these values according to its bit positions and then add up

corresponding decimal values to get the value.

128 64 32 16 8 4 2 1

1 1 1 0 0 1 0 0

= 128 + 64 + 32 + 4 = 228

Please do the following examples yourself to clarify the concepts:

Convert: 192.168.140.20 in binary.

Convert: 11110011 in decimal.

After learning conversion between decimal and binary notations, we will turn our focus to private and public IP

addresses and classes of IP Addresses.

5

Public and Private IP Addresses

Continuing our IP addressing discussion. IP addresses can be further divided into Private IP addresses and Public

IP Addresses. To preserve IP address space Private IP Addresses were introduced. Private IP addresses are used

on the internal network and never advertised to the public network. Private IP addresses are defined in below

mentioned ranges:

10.0.0.0 ‐ 10.255.255.255 Addresses: 16,777,216 172.16.0.0 ‐ 172.31.255.255 Addresses: 1,048,576 192.168.0.0 ‐ 192.168.255.255 Addresses: 65,536

Private IP addresses go through a process of NATing if they want to communicate with Public Internet.

Public addresses are those addresses which are advertised on the public network, inter‐networks etc.

Classes of IP Addresses Several classes of IP addresses have been defined for Network identification and network address assignment

according to design requirements. For these classes numeric ranges were defined, each range can be used for a

specific number of hosts and network addresses. IP address classes are: A, B, C, D, E. Each class has its own Host

and Network Ranges. The IP address classes were developed keeping in mind: to accommodate large

companies with a lot of host requirements and small companies with minimum host requirements!

The normal range used mostly in public network is Class A, B and C. Class D and Class E are used for special

purposes.

Class D: this range IP addresses are used for Multicast addressing requirements.

Class E: this range is reserved for research and scientific purposes.

Before moving forward into this class discussion, let us discuss one more important aspect of IP addressing. IP

addressing is a hierarchical design. The telephone number system is the best example of a hierarchical design

model. A telephone number is composed of Country Code, Area Code, and local exchange code. The same is

true for an IP address. An IP address is made of two parts, one part is called the Network Portion and the second

part is called the Host Portion. The Network portion of the IP address is used to keep track of the domain to

which some specific host belongs and the host portion of the IP address is used to trace the machine or

computer node.

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Below we will discuss Class A, B and C in more detail.

Class A IP Addresses:

The first octet of the Class A address is composed of Network Portion and its most significant bit is always off. All

other three octets denote the host portion. Simply we can say:

N.H.H.H

0xxxxxxx.H.H.H

If we want to calculate the range of Class A IP addresses, we can move as:

00000000.H.H.H (0.H.H.H)

01111111.H.H.H (127.H.H.H)

If the 1st portion of an IP address is in range (0‐127), then that IP address belongs to Class A! But as you know

127.0.0.1 range is reserved for loop back interface and we can’t use it for Class A and also not use an IP address

starting from 0 , then the revised range would be from (1‐126)! A few examples of Class A IP address are:

10.0.0.1

100.2.3.1

110.130.13.4

123.4.1.110

Class B IP Addresses:

The first two octets of Class B IP address are composed of Network Portion, and the other two octets are

composed of Host portion, in doted representation it can be given as:

N.N.H.H

The most significant two bits in the first octet are kept 10,

10xxxxxx. xxxxxxxx. H. H

So the range of Class B IP address space can be calculated from its first octet as follow:

10000000 to 10111111 (128 – 191)

Some examples of Class B IP addresses are:

130.50.3.3

170.16.3.1

172.31.3.3

7

Class C IP Addresses:

The first three 8 bit portions of a class C IP address are composed of Network Portion, and the last one denotes

the host portion. It can be simplified as:

N.N.N.H

The three most significant bits are kept 110 despite all bit position changes. So the range for Class C IP address

space can be calculated as:

110xxxxx. xxxxxxxx. H. H

11000000‐ 11011111

192 – 223

Some examples of Class C IP address are:

192.168.100.3

220.221.120.135

210.49.66.110

All of the above discussion regarding IP address Classes can be summed up in below table:

IP Address 1st Octet Range Usable Network and Host IDs

Class A (N.H.H.H) 1‐126 Networks : 2^8‐2 and Hosts= 2^24‐2

Class B (N.N.H.H) 128‐191 Networks : 2^16‐2 and Hosts= 2^16‐2

Class C (N.N.N.H) 191‐223 Networks : 2^24‐2 and Hosts= 2^8‐2

Two more ranges for your technical mind:

D: Multicast range: 224 – 239 (Examples: 224.0.0.9)

E: IP Address range for R&D: 240 – 255 (Example: 241.0.0.9)

8

Subnet Mask:

We will end this discussion with Subnet mask. A subnet mask is used by routers and end machines to check, to

which network, the host belongs. The network ID of the IP address is calculated by Logical ANDING of the Subnet

mask with the IP Address. Each Class has its own subnet mask:

Class A Subnet Mask is: 255.0.0.0 and is also denoted by /8

In binary: 11111111.00000000.00000000.00000000

Class B Subnet Mask is: 255.255.0.0 and is also denoted by /16

In binary: 11111111.11111111.00000000.00000000

Class C Subnet Mask is: 255.255.255.0 and is also denoted by /24

In binary: 11111111.11111111.11111111.00000000

Sub netting: One of the most important topics in Computer Networks and CISCO realm is sub netting. The main motivation

behind sub netting was the best utilization of the scarce resources of available IP addresses. In simple words,

sub netting is the process of taking a single Network address and creating further smaller Network IDs from it,

called Subnets (Sub Networks). In the process of sub netting , bits can be borrowed from the host portion of an

IP Address, the borrowed bits are added to the Subnet Mask of that IP address. We will further clarify the sub

netting process via different examples. The main goal behind sub netting a given network address is to create

our required number of smaller network IDs and to achieve our desired number of hosts per subnet ID.

9

Sub Netting a Class C IP Address We learned what a sub net mask is, and what is sub netting. Now we will learn how to sub net. The basic Sub

netting process starts from below mentioned questions:

How many subnets are required?

How many hosts per subnet are required?

Compute the effective subnets?

Compute the valid host IP Addresses?

For keeping the sub netting process simple we will stick to these questions for time being, will further add up

things as per demand.

The anatomy of a typical Class C address is:

N.N.N.H with subnet mask 255.255.255.0 or /24

Suppose we have an IP address: 192.168.10.0 /24 and our network design requirement is 8 subnets!

For 8 subnets, how many bits we can take from the host portion (last octet) of the given IP address? For this, just

do a mental calculation using below formula:

2^y = 8, two to the power which value can give us 8? Simply

2^3 = 8, great! For getting 8 subnets, 3 bits can be borrowed from the host portion of the given IP address

(192.168.10.0), the borrowed bits are moved to the given subnet mask:

11111111.11111111.11111111.11100000

The new Subnet mask is: 255.255.255.224 in CIDR Notation /27!

Now the mystery of hosts per subnet! As we have borrowed three bits from the last octet of the host portion,

how many bits are remaining? 5! Yes, you are right, 5 bits are remaining. So the number of usable hosts per

subnet will be given as:

2^5‐2 = 32 – 2 = 30

From above two steps, we have achieved two tasks:

We will have 8 subnets and there will be 30 usable host addresses per subnet!

3 borrowed

10

Okay, now the tricky part, what are the valid subnets block size? Please keep this formula in mind:

Subnet block size = 256 – subnet mask modified octet.

As we have new subnet mask 255.255.255.224, the modified octet is the last one (224), so

Subnet block size = 256 – 224 = 32

So our subnet block will start from 0, 32, and 64 and will go on for increment of 32. So our 10 new subnets are:

192.168.10.0

192.168.10.32

192.168.10.64

192.168.10.96

192.168.10.128

192.168.10.160

192.168.10.192

192.168.10.224

All the valid hosts and IP ranges given by each subnet can be summarized in below table:

IP Address Network Address 1st Host Address Last Host Address Broadcast Address

192.168.10.0 192.168.10.0 192.168.10.1 192.168.10.30 192.168.10.31

192.168.10.32 192.168.10.32 192.168.10.33 192.168.10.62 192.168.10.63

192.168.10.64 192.168.10.64 192.168.10.65 192.168.10.94 192.168.10.95

192.168.10.96 192.168.10.96 192.168.10.97 192.168.10.126 192.168.10.127

192.168.10.128 192.168.10.128 192.168.10.129 192.168.10.158 192.168.10.159

192.168.10.160 192.168.10.160 192.168.10.161 192.168.10.190 192.168.10.191

192.168.10.192 192.168.10.192 192.168.10.193 192.168.10.222 192.168.10.223

The usable host portion for each octet is highlighted! And we are done with sub netting for Class C! Was it

simple? No, you will need some practice to get the full command on it Now we can use the above mentioned

IP plan, in our network design, a single IP has been converted into 8 usable sub networks and each network

having 30 host capacity, isn’t it amazing?

11

Sub netting a Class B Address:

We will use the method explained previously to subnet a Class B address and a class A address. The network

design requirements are the same as above (i.e. 8 sub networks required):

Given Class B Address is: 172.16.0.0

Default Class B Mask: 255.255.0.0

How many host bits needed? 3! Yes absolutely right. Okay now we are going to embed these 3 bits in the Class B

mask:

11111111.11111111.00000000.00000000

11111111.11111111.11100000.00000000

The modified Subnet mask is

255.255.224.0 /19

So what’s next? Yeah, you got it,

Subnet block size = 256 – subnet mask modified octet


As we have taken bits from 3rd octet, our new subnets are:

172.16.0.0 – 172.16.32.0 – 172.16.63.0 – 172.16.95.0 – 172.16.127.0 ‐ ‐ ‐ ‐ ‐ > 172.16.224.0


172.16.0.0 172.16.0.0 172.16.0.1 172.16.31.254 172.16.31.255

172.16.32.0 172.16.32.0 172.16.32.1 172.16.62.254 172.16.62.255

172.16.63.0 172.16.63.0 172.16.63.1 172.16.94.254 172.16.94.255

172.16.95.0 172.16.95.0 172.16.95.1 172.16.126.254 172.16.126.255

172.16.127.0 172.16.127.0 172.16.127.1 172.16.158.254 172.16.158.255

172.16.159.0 172.16.159.0 172.16.159.1 172.16.190.254 172.16.190.255

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As only 3 bits were reserved, the number of usable hosts per subnet is:

Usable hosts per subnet = 2^13‐2 = 8190! (8190 hosts/subnet)

Sub netting Class A Address:

If you have mastered Class B and Class C sub netting then Class A is not that hard! The network design

requirements are the same as above (i.e. 8 sub networks required) and we have a Class A IP address of 10.0.0.0:

Given Class A Address is: 10.0.0.0

Default Class B Mask: 255.0.0.0

How many host bits needed? 3! Yeah that’s right. Okay now we are going to embed these 3 bits in the Class A

mask:

11111111.00000000.00000000.00000000

11111111.11100000.00000000.00000000

The modified Class A mask is

255.224.0.0 /11

Pretty easy!

As we have modified our second octet in the Subnet Mask of Class A, so it will be subtracted only from 256, so:

Subnet block size = 256 – subnet mask modified octet


So our new subnets are:

10.0.0.0 ‐ 10.32.0.0 ‐ 10.64.0.0 – And so on

The feel of the 8 subnets would be best visible in the tabular form as follows :


10.0.0.0 10.0.0.0 10.0.0.1 10.31.255.254 10.31.255.255

10.32.0.0 10.32.0.0 10.32.0.1 10.63.255.254 10.63.255.255

10.64.0.0 10.64.0.0 10.64.0.1 10.95.255.254 10.95.255.255

10.96.0.0 10.96.0.0 10.96.0.0 10.127.255.254 10.127.255.255

10.128.0.0 10.128.0.0 10.128.0.1 10.159.255.254 10.159.255.255

10.160.0.0 10.160.0.0 10.160.0.1 10.191.255.254 10.191.255.255

10.192.0.0 10.192.0.0 10.192.0.1 10.223.255.254 10.223.255.255

10.224.0.0 10.224.0.0 10.224.0.1 10.255.255.254 10.255.255.255

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Believe me, by just looking at the above given examples, you will be frightened by sub netting, but if you actually

begin practicing them, then you will realize that how easy sub netting is. So don’t give up, reread the above

examples, you will find plenty of sub netting problems online. Remember, only Practice and more Practice are

the key to success in sub netting.

One very interesting tool while practicing Subnetting is Solar Winds, Advance Subnet Calculator. You can

download it and verify your sub netting from it. For example, for above Class A Subnetting, the Solar Winds

Subnetting Calculator output is:

This sub netting tool is awesome and you will love it!

14

Sub netting Exercise

We have learned a fair bit about sub netting, its time to do some practice. In order to challenge your mind,

please do the following subnet exercises:

Class C:

We have an IP of 192.168.2.0 /24; our network design requirement is 16 subnets!

We have an IP of 192.168.150.0 /24; our network design requirement is 14 hosts per network!


Class B:




Class A:




15

VLSM (Variable Length Subnet Masking):

We have a very scarce resource of IP v4, that’s why private addresses were created, that’s why sub netting was

introduce and that’s why NATing is done to preserve the IP addresses. What if we design our network carelessly

and waste many precious IP addresses? This can happen if we don’t take precautions in network design and

don’t use VLSM. VLSM give us a facility to use different subnet mask networks in our topology, and believe me

VLSM can save us a lot of address space. To understand this further, please consider the following network:

Suppose we are using 192.168.10.0 Class C Address in above network and have made 8 subnets like:


192.168.10.0 192.168.10.0 192.168.10.1 192.168.10.30 192.168.10.31

192.168.10.32 192.168.10.32 192.168.10.33 192.168.10.62 192.168.10.63

192.168.10.64 192.168.10.64 192.168.10.65 192.168.10.94 192.168.10.95

192.168.10.96 192.168.10.96 192.168.10.97 192.168.10.126 192.168.10.127

192.168.10.128 192.168.10.128 192.168.10.129 192.168.10.158 192.168.10.159

192.168.10.160 192.168.10.160 192.168.10.161 192.168.10.190 192.168.10.191

192.168.10.192 192.168.10.192 192.168.10.193 192.168.10.222 192.168.10.223

Do you feel anything wrong with above network? Hmm, apparently there is nothing wrong according to

addressing point of view in above network but we are wasting a lot of IP addresses. How? Okay look at the serial

connection of Router0 and Router1, we are using a subnet of 192.168.10.32/27, this subnet can give us the

following host addresses:

192.168.10.33

192.168.10.34

192.168.10.35

192.168.10.36

192.168.10.37

192.168.10.38

‐‐‐‐‐ Till 192.168.10.62!

16

Suppose we assign 192.168.10.33 and 192.168.10.34 to our Connected WAN interfaces, what about the rest of

28 addresses? They are simply wasted! The same thing is happening on our Switch0 and Switch4, we are

allocating more addresses than actually required! So how can we protect these IP addresses from getting

wasted? That’s where VLSM comes handy.

VLSM Practical Example

By using a different subnet mask for each router interface, we can create the IP addresses according to the

network requirements. Like for the WAN interface only two IP addresses are required. So if we use a mask of /30

for this Class C address 192.168.10.0, we can have 2 host bits, which are 2^2‐2 = 2 hosts per subnet! Any subnet

from /30 mask will full fill our requirements of two IP addresses for the two connected serial WAN interfaces.

So if we take 192.168.10.0 and use 255.255.255.252 as the subnet mask, we can have

Subnets: 2^6 = 64 subnets, an 2^2‐2 = 2 hosts /subnet

Block Size = 256‐252 = 4. So 0, 4, 8, 12, ‐‐‐‐


192.168.10.0 192.168.10.0 192.168.10.1 192.168.10.2 192.168.10.3

192.168.10.4 192.168.10.4 192.168.10.5 192.168.10.6 192.168.10.7

192.168.10.8 192.168.10.8 192.168.10.9 192.168.10.10 192.168.10.11

192.168.10.12 192.168.10.12 192.168.10.13 192.168.10.14 192.168.10.15

192.168.10.16 192.168.10.16 192.168.10.17 192.168.10.18 192.168.10.19

192.168.10.20 192.168.10.20 192.168.10.21 192.168.10.22 192.168.10.23

Continues Continues Continues Continues Continues

192.168.10.252 192.168.10.252 192.168.10.253 192.168.10.254 192.168.10.255

We have chosen below address for our WAN Connection:


192.168.10.4 192.168.10.4 192.168.10.5 192.168.10.6 192.168.10.7

Okay, if we take a careful look on our switches, Switch0 requirement is 10 hosts, so we need to create a subnet

according to this requirement. If we take 4 bits for our subnet, we are remaining with 4 host bits, which are

sufficient to fulfill our requirements because 2^4‐2 = 14 hosts! So repeating the above process, a subnet mask of

/28 is enough for this:

So if we take 192.168.10.0 and use 255.255.255.240 as the subnet mask, we can have

Subnets: 2^4 = 16 subnets, an 2^4‐2 = 14 hosts /subnet

Block Size = 256‐240 = 16. So 0, 16, 32, 48, ‐‐‐‐

17

These subnets can be summarized as:


192.168.10.0 192.168.10.0 192.168.10.1 192.168.10.14 192.168.10.15

192.168.10.16 192.168.10.16 192.168.10.17 192.168.10.30 192.168.10.31

192.168.10.32 192.168.10.32 192.168.10.33 192.168.10.46 192.168.10.47

192.168.10.48 192.168.10.48 192.168.10.49 192.168.10.78 192.168.10.79

192.168.10.80 192.168.10.80 192.168.10.81 192.168.10.94 192.168.10.95

Continues Continues Continues Continues Continues

192.168.10.239 192.168.10.239 192.168.10.240 192.168.10.254 192.168.10.255

As we have already used 192.168.10.4/30 subnet for WAN, we can use the below IP subnet from /28 mask to

fulfill our requirement:


192.168.10.16 192.168.10.16 192.168.10.17 192.168.10.30 192.168.10.31

And we can use the same subnet (/28) to fulfill our IP addressing requirements on Switch4; it will not only

preserve the IP addresses but will also give us room for a few more IPs, if we expand our network in the future.

We will use the following IP addresses for Switch4:


192.168.10.32 192.168.10.32 192.168.10.33 192.168.10.46 192.168.10.47

So our network with VLSM will look like:

The above network, with different Subnet masks, is known as a Classless Network! And to route packets on such

a network we use routing protocols which support classless networks. RIPv2, EIGRP, and OSPF support classless

networks. Using VLSM, care should be taken to avoid IP address conflict. This can be achieved via proper

documentation of the IP addressing scheme used in the network or any method that seems convenient for you.

But a proper documentation is always recommended!

Please remember: in VLSM , always start from the largest host requirement network!

18

Super netting or Route Summarization Route summarization is all about minimizing processing load of the routers and for the robust operation of networks. Via route summarization, instead of advertising individual network IDs, a router advertises summary of the similar subnets. By sending this summary of the addresses, the neighbor routers are able to send data to all the networks related to the summary advertised. That summary route is a sort of superset of all the networks for which we have configured it, that’s why it’s also called Supernetting. The other common term for route summarization is Route Aggregation, used in BGP! There could be various route summaries for the given set of IP addresses, but the best summary is the one which narrows down the IP address space as much as possible. Let’s understand this via an example:

Let suppose the following addresses are connected to our Corporate router:

192.168.16.0/24

192.168.17.0/24

192.168.18.0/24

192.168.19.0/24

172.1.4.0/24

172.1.5.0/24

172.1.6.0/24

172.1.7.0/24

If the router advertises each address individually to its connected routers, it will increase routing processing not

only on our Corporate router but also on the connected routers. As each router had to parse individual IP

addresses, also maintaining big routing table is a hefty task for a router.

19

In the given network, we have a total of 8 Network Addresses in the routing table. We will use route

summarization to summarize these addresses into only two addresses of 192.168.X.X address space and

172.1.X.X! So how can we create a summary route for the first block i.e.:

192.168.16.0/24

192.168.17.0/24

192.168.18.0/24

192.168.19.0/24

Hmm, in above address, first two octets are same, converting our addresses into binary:

Address 1st Octet 2nd Octet 3rd Octet 4th Octet

192.168.16.0 11000000 10101000 00010000 00000000

192.168.17.0 11000000 10101000 00010001 00000000

192.168.18.0 11000000 10101000 00010010 00000000

192.168.19.0 11000000 10101000 00010011 00000000

Subnet Mask 255 255 252 0

Okay, 1st and 2nd Octets are similar, and first 6 bits (highlighted in red) are similar in 3rd Octet. Our summary

address would be the lowest of the addresses i.e. 192.168.16.0 and our modified subnet mask would be /22 i.e.

255.255.252.0 (the 6 similar bits in the 3rd octet are considered as 1 while calculating the revised subnet)! The

same process can be repeated for the Class B Address:

172.1.4.0/24

172.1.5.0/24

172.1.6.0/24

172.1.7.0/24

Address 1st Octet 2nd Octet 3rd Octet 4th Octet

172.1.4.0 10101100 00000001 00000100 00000000

172.1.5.0 10101100 00000001 00000101 00000000

172.1.6.0 10101100 00000001 00000110 00000000

172.1.7.0 10101100 00000001 00000111 00000000

Subnet Mask 255 255 252 0

So our Summary route is: 172.1.4.0/22 or 172.1.4.0 255.255.252.0. Once these summary routes are configured

on our above network, it would advertise all the networks connected to it in the form of following super netted

IP Addresses:

192.168.16.0/22 & 172.1.4.0/22

Route summarization is one of the main feature in OSPF, the routes are summarized at the ABRs (Area border

routers) and advertised to the backbone area by all other connected regular areas. BGP also uses the route

aggregation. Care should be taken while implementing route summarization, we should try to minimize the risk

of creating room for IP addresses which doesn’t belong to our network. If care is not considered, we will begin

getting packets for the addresses which are not configured in our network.

easy ip addressing and subnetting manual for starters

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