FreeVLSM eBook

VLSM Made Easy Chika Nwokeoma

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Variable Length Subnet Masks

Made Easy

(VLSM)



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Contents ………………………………………………………………………………………..

IP Addressing Overview.....4 . What an IP Address?.......4

. IP Address Format..........5

. IP Address Class and Range......6 Planning to Address the Network....8 . Why Plan, design and document....8

The Use of a hierarchal or structural IP Addressing Scheme...9 . Benefits of a hierarchical network design...9

The use of Subnet in Structuring the Network....12 . Let’s talk Subnet Mask.....12 Humans see Decimals; Computer sees Binary.....13

. IP Address Classes, decimals and binary Summarized....14 Basic Subnetting Process....21 Variable Length Subnet Masks (VLSM)....24

Benefits of VLSM:....24 Implementing VLSM....25 VLSM Practice....33

0101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101 00101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101 0101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101



IP Addressing Overview My aim of writing this eBook is to teach you how to subnet using VLSM the easy way. I

will not go into those technical jargons; I will try as much as possible to use a layman’s language to pass my message through. For those of us who expect CCNA technical

jargons; forgive me, I follow and work out everything in live my own way and learn

jargons later!

I promised myself to go straight to VLSM, but I won’t be doing some readers justice without giving an overview of what IP addressing is. If you are writing the CCNA exam

or you are a network administrator, the thorough knowledge of TCP/IP especially IP addressing and subnetting is a must!

What an IP Address?

An IP addressing is a sort of unique identification number used in the location of each device on the network or internet. It’s like a phone number, a house number, or post

code used to identify a house or location of an individual. (Those of us who work in a post office know how it works.).

For your letter to reach its location, it must be addressed properly with the destination

country , when it get there, the post office will narrow it down to the state or province, town, village and finally the street and house number then to the intended individual.

With this house number, you can send and receive letters. The communication between hosts or devices (PCs, server, printers) on different

network is made possible through IP addresses.. Before we go further, let’s see how to find your ip address of your computer.

The easiest way is to use the command prompt; for other ways see ways to find your IP

address.

In the command prompt; (Windows 7, Vista, and XP)

Click Start, and then select All Programs.

Click Accessories, and then select Command Prompt.

At the command prompt, enter: ipconfig Tap the enter tab.

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The IP address of your Computer will be displayed with the network mask and dns

server ip as the case may be

Now…what about the subnet mask; I could hear you ask, don’t worry, we will go fully into that as we proceed.

A proper IP address is divided into two parts: The Host Address and The Network

Address. The host part of the address identifies the individual nodes or machines. (Just as a

number identifies a house in a street) While he network address portion identifies the

network or network segment on which the host or machines are located. (Just as the post code locates a town or village where a house is)

IP addresses are divided into three main classes (five actually), these are Class A, Class B and Class C.

Then, classes D and E..

(Class D addresses are used for multicasting, and Class E addresses are reserved for

testing future use.)

IP Address Format

A usable IP address is 32-bit (binary) which is broken up into four octets (parts), and is

set into a dotted−decimal notation pattern.

An octet is a set of 8 bits. Below is an example of an IP address with its four octets arranged into its dotted−decimal pattern:

192.168.10.41



The diagram below will help you understand the classes, bits and octets, if it doesn’t,

don’t worry, as we go on; you will. Just stay with me.

IP Address Class and Range:

Class A:

Class A addresses Class A IP addresses use the first 8 bits (first octet) to designate the network address. The other 24 bits or (Three Octets) are for the host address. There are 16,777,214 host addresses available in a Class A address.

Class B IP addresses

Class B addresses use the first 16 bits (two octets) for the network address. The last two octets are used for the host address. The first 2 bits, which are always 1s and 0s,

designate the address as a Class B address, and 14 bits are used to designate the network. This leaves 16 bits (two octets) to designate the hosts.



Class C IP addresses

Class C addresses use the first 24 bits (three octets) for the network address, and only the last octet for host addresses. The first 3 bits of all class C addresses are set to 1 1

0, leaving 21 bits for the network address, which means there can be 2.097,150 Class C networks, but only 254 hosts per network.

IP Address Classes Summary Table

Class Address Range Number of

Networks

(Bits)

Number of Hosts

(Bits)

Number of Hosts/Network

A 1-126 8 24 16,777,214

B 128-191 16 16 5,534

C 192-223 24 8 254

... If you are writing the CCNA

exam or you are a network

administrator, the thorough

knowledge of TCP/IP especially IP

addressing and subnetting is a

must!



Planning to Address the Network

It’s a well recommended practice for a network administrator not to randomly allocated

address space in an organization’s network. A well designed or structured network address provides network scalability and should be planned and documented.

Why Plan, design and document?

Planning, design and documenting your network IP addressing helps in the

Prevention of duplication of addresses i. Prevention of duplication of addresses: To err is human they say, so by

proper planning for network addressing and design, each host in the organization network must be allocated with a unique IP address. This

prevents the assignment of an IP address to more than one host. ii. It ensures in the Proper control access to the network: By planning and

documenting of allocation of addresses, the security and accessibility of

network devices such as servers are easily controlled. For example, if an organization server that provides resources to internal or external network is

randomly addressed, blocking access to its address is difficult and clients may not be able to locate this resource

iii. It helps in the Monitoring of network security and performance: Similarly, we need to monitor the security and performance of the network hosts and the

network as a whole. As part of the monitoring process, we examine network traffic looking for addresses that are generating or receiving excessive packets.

If we have proper planning and documentation of the network addressing, we can identify the device on the network that has a problematic address.

Some examples of different types of hosts are:

• End devices for users (PCs, laptops, PDA, etc.)

• Servers and peripherals

• Hosts that is accessible from the Internet (servers) • Intermediary devices (switches, bridges, hubs, access points etc.)

Each of these network devices should be assigned with a logical block of addresses which has to fall within the address range of the organization’s network.

Each network devices should be assigned with a

logical block of addresses which has to fall

within the address range of the organization’s

network.



The Use of a hierarchal or structural IP Addressing Scheme

A hierarchical addressing structure logically groups networks into smaller subnetworks

known basically as subnets.

An effective hierarchical address structure consists of a classful network address in the Core Layer of the organization’s network; then is subdivided or sequentially broken

down into smaller subnets in the Distribution and Access Layers.

This type of addressing structure helps in simplifying network management and troubleshooting and also improves scalability and routing performance. Unlike a flat IP

addressing design which does not scale well in a large network.

Using a well-structured or hierarchical IP addressing scheme for a network makes it easier to increase the size of the network. A larger network can accommodate more

users, remote branches or offices. Also properly designed hierarchical IP addressing

scheme also makes it easier to perform route summarization

Benefits of a hierarchical network design

A well-structured network with the proper allocation and deployment of IP address

blocks has the following features: • Routing stability

• Service availability • Network scalability

The diagram below shows two examples; one representing a non-hierarchical and the

other a hierarchical addressing. Both examples use the same network topology. Only the addressing scheme changed.

Switched LAN's at the Access Layer connect to a Distribution Layer router, which connects to a Core Layer router, which in turn connects to the internet.

An effective hierarchical address structure

consists of a classful network address in the

Core Layer of the organization’s network;

then is subdivided or sequentially broken

down into smaller subnets in the

Distribution and Access Layers.



In the non-hierarchical addressing assignment, each network IP addresses is not related, as follows:

Connection to the internet: 192.168.10.0

Core to Distribution Layer: 192.168.5.0 LAN 1: 10.10.1.0

LAN 2: 172.17.10.0



In the hierarchical network addressing design, there is a logical grouping of networks address: Connection to the internet: 172.16.0.0 /16 Core to Distribution Layer: 172.17.1.0 /24 LAN 1: 172.17.1 .32 /27 LAN 2: 172.17.1.64 /27



The use of Subnet in Structuring the Network

Why use subnet?

Subnetting helps to ensure the smooth flow of network traffic between hosts on the network. More especially, it helps in the proper management of IP address. By

organizing hosts into logical groups, subnetting can improve network security and performance.

These are some of the reasons to divide the network into subnets: • Physical location

• Logical grouping • Security

• Application requirements • Broadcast containment

• Hierarchical network design

Let’s talk Subnet Mask

In order to use subnetting to structure the network, it is very important to have a clear

understanding of the structure of the subnet mask.

The subnet mask shows where or location of hosts in a network.

The subnet mask is a 32-bit value that differentiates between the network bits and the host bits. It consists of a string of 1s followed by a string of 0s. The 1 bit represents the

network portion and the 0 bits represent the host portion. To get clearer perspective of this, look back at the diagram on IP addressing overview.

Below are the IP address classes’ subnet masks:

• Class A addresses use a default subnet mask of 255.0.0.0 or a slash notation

(CIDR) of /8 • Class B addresses use a default mask of 255.255.0.0 or /16

• Class C addresses use a default mask of 255.255.255.0 or /24

The 255 part of it represents the network portion and the 0 parts represents the host portion

The subnet mask shows where or

location of hosts in a network…

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Humans see Decimals; Computer sees Binary.

Ok, this might sound out of place, but emphatically, what we see is different from how

the computer sees it. For example; right click on any webpage, click on the “view source” from the pop up menu window and you get to see how computer sees the

webpage you are seeing!

Same goes to IP addressing and subnet masks. Humans see decimals, but computer sees and calculates in binary.

Firstly, below is a cram table as I called it, this table will give you a quick access,

especially on the day of your CCNA exam or when addressing a subnetwork.

The cram table summarized all the classes A to C.

Subnetting on different classes is the same process; the only difference is on the octet you are working on. To subnet a class; you need to work on the host bits of that class:

Class A = Second octet

Class B = Third octet Class C = Fourth octet

SUBNET CRAM TABLE

Bit Value 128 64 32 16 8 4 2 1

Bit Borrowed 1 2 3 4 5 6 7 8

Usable host address

126 62 30 14 6 2

Subnet Mask 128 192 224 240 248 252 255 256

Subnet Prefix/CIDR

/25 /26 /27 /28 /29 /30

To get the network address, you add the bit value borrowed to get the subnet mask.

Example: When you borrow 1 bit = 128 value

So your network address subnet mask will be 255.255.128.0 (Class B).

If you had borrowed 2 bits = 128+64 = 192

Your network address subnet mask will be

255.255.192.0. etc.etc



IP Address Classes, decimals and binary Summarized

Is not enough to pass the CCNA exam, anyone can do it these days by purchasing

question dumps. But how will tour performance be in the real world!

I know this part will be somehow difficult and boring, but if you want to be good in your subnetting; you need to learn the breakdown below; in detail about

decimal and binary conversion.

Study the decimal and binary parts carefully; you will notice how bits are turned

on from 0 to 1 to turns it into a network: (remember the Subnet mask determines the location of the network). Study this from time to time, is no

rocket science!!

Class A (Network. Host. Host. Host) subsequently, in order to subnet, we went

on to borrow a bit from the host bit to turn it on into a network address.

Remember, in class A, you are working on the FIRST octet.

Example of Class A IP Address: 10.0.0.0

Default dotted decimal Subnet Mask: 255.0.0.0

Subnet Mask in Binary: 11111111.00000000.00000000.00000000

Slash Notation or CIDR: /8

Number of host bits: 24 (counting the 0s)

Hosts Possible, 2 to the power of n minus 2: 16777214

One bit is borrowed from the host part below: (see the bold 1s)

Dotted decimal Subnet Mask: 255.128.0.0



Number of host bits: 23




Two bits are borrowed from the host part below:






Three bits are borrowed from the host part below:






Four bits are borrowed from the host part below:








Five bits are borrowed from the host part below:


Subnet Mask in Binary: 11111111.11111000.00000000.00000000>




Six bits are borrowed from the host part below:






Seven bits are borrowed from the host part below:








Class B

Example IP address: 172.16.0.0 (Network. Network. Host. Host

Borrow bits from the host part to turn it on or into a network address. Remember, in class B, you are working on the SECOND octet.

Default dotted decimal Subnet Mask: 255.255.0.0 Subnet Mask in Binary: 11111111.11111111.00000000.00000000

Slash Notation or CIDR: /16 Number of host bits: 16


One bit is borrowed from the host part below:

Dotted decimal Subnet Mask: 255.255.128.0 Subnet Mask in Binary 11111111.11111111.10000000.00000000




Dotted decimal Subnet Mask: 255.255.192.0 Subnet Mask in Binary 11111111.11111111.11000000.00000000




Dotted decimal Subnet Mask: 255.255.224.0 Subnet Mask in Binary: 11111111.11111111.11100000.00000000










Five bits are borrowed from the host part below:


Subnet Mask in Binary: 11111111.11111111.11111000.00000000 Slash Notation or CIDR: /21





Subnet Mask in Binary 11111111.11111111.11111100.00000000 Slash Notation or CIDR: /22

Number of host bits: 10 Hosts Possible, 2 to the power of n minus 2: 1022

Seven bits are borrowed from the host part below:







From here is Class C, (Network. Network. Network. Host)

In class C, you borrow bits from the host part to turn it on or into a network address. Remember, in class C, you are working on the THIRD octet.

Default dotted decimal Subnet Mask: 255.255.255.0 Subnet Mask in Binary: 11111111.11111111.11111111.00000000



One bit is borrowed from the host part below:


















Five bits borrowed from the host part below:









Humans see decimals, computer sees and

calculate in binary.

Subnetting on different classes is the

same process; the only difference is on

the octet you are working on. To subnet a

class; you need to work on the host bits

of that class.



Basic Subnetting Process Using the cram table makes subneting on any of the classes easier.

When using a hierarchical addressing scheme, the subnet mask reveals so much

information about the IP and network address.

For example, an IP address of 192.168.10.45 /27 shows the following information:

Given the above IP address, with the slash notation or CIDR, you will know (following the cram table) that /27 means the subnet mask will be 255.255.255.224.

This means that 3 additional host bits were borrowed with the bit value of 32 and this creates 6 subnets (2^3 = 6). Now this will accommodate 30 usable host addresses (32,

but 2 reserved for network and broadcast)

Bit Value 128 64 32 16 8 4 2 1

Bit Borrowed 1 2 3 4 5 6 7 8

Usable host address 126 62 30 14 6 2

Subnet Mask 128 192 224 240 248 252 255 256

Subnet Prefix/CIDR /25 /26 /27 /28 /29 /30

The network address in this case will be:

Network Address: 192.168.10.32 Subnet mask: 255.255.255.224

Usable host addresses: 192.168.10.33 to 192.168.10.62 Broadcast address: 192.168.10.63

192.168.10.64 starts the network address of the next subnet.

Note: if you are wondering how we got the 224 in the

fourth octet of the subnet mast; we added the bits

value we borrowed: 128+64+32 = 224 or /27

Each time a bit is borrowed, you have to minus

2(each for network and broadcast address) from

the bit value to obtain the number of usable IP

address for hosts



Example 2

Giving the IP address of 192.168.20.72 /26

What is the network address, subnet mask, usable hosts addresses and broadcast address.

Bit Value 128 64 32 16 8 4 2 1

Bit Borrowed 1 2 3 4 5 6 7 8


Subnet Mask 128 192 224 240 248 252 255 256


If you look at the cram table, /26 means 2 bit value was borrowed from the host bits of the network to create a subnet.

Bits value: 128+64=192.

So the subnet mask will be 255.255.255.192

Network address: 192.168.20.64 Usable hosts addresses: 192.168.20.65 to 192.168.20.126

Broadcast address: 192.168.20.127

Next network address will be: 192.168.20.128

If we have to continue:

Network address: 192.168.20.128 /26

Usable host range: 192.168.20.129 to 192.168.20.190 Broadcast address: 192.168.20.191

Network address: 192.168.20.192 /26 Usable host range: 192.168.20.193 to 192.168.20.254

Broadcast address: 192.168.20.255



Refresher:

Basic subnetting does not scale well in a large network. A

subnet of /26 is required to accommodate only network

segment of 58 hosts. Using a basic subnetting scheme is

not only wasteful, but creates only four subnets. But with

VLSM concept, you can subnet a subnet. VLSM allows the

use of different masks for each subnet.



Variable Length Subnet Masks (VLSM) VLSM basically is subnetting a subnet. This concept of subnetting a subnet address,

provide for efficient use of address space. With VLSM, IP network addressing is structured in hierarchical form, this enables the

router to summarize routes in the routing table.

Route summarization helps to reduce the size of routing tables in distribution and core

routers, and the Smaller the routing tables, the less CPU time for routing lookups.

Don’t get me wrong, basic subnetting is only works well for smaller networks but does not offer the flexibility needed in larger networks.

Most classless routing protocols support the use of VLSM because the subnet mask is

sent with all routing update packets. Classless routing protocols include RIPv2, EIGRP, and OSPF.

Benefits of VLSM: • Allows efficient use of address space

• Allows the use of multiple subnet mask lengths

• Breaks up an address block into smaller blocks • Allows for route summarization

• Provides more flexibility in network design • Supports hierarchical enterprise networks

VLSM basically means, subnetting a subnet.

This concept of subnetting a subnet address,

provides for efficient use of address space.



Implementing VLSM #1

To really understand how VLSM works, you need to be clear on how block sizes work to create a VLSM subnet masks.

For example, when you have host requirement of 52, then you will need the block size

of 64, if you require 13 host, you have to work with block size 16.

Even if you need 45 hosts, block size of 64 will have to be your choice. You don’t cut corners by trying to create or make up block sizes! That’s why the cram table is so

effective.

Example:

Let’s use the topology below as an example on how you can use VLSM to break up a

subnet into smaller portions for use on serial links so as to save IP address wastage, mind you each network is has an equal host range of 30:

Given the subnet address: 192.168.10.0 /27

Firstly, we bring the Modified cram table and refer to it as we go along:

Bit Value 128 64 32 16 8 4 2 1

Bit Borrowed 1 2 3 4 5 6 7 8


Subnet Mask 128 192 224 240 248 252 255 256


As we move along, use the cram table as guidance

There are four routers, R1, R2, R3, and R4.

R1 is connected to R2 via Serial link (Subnet Address: 192.168.10.192 /30). R2 is connected to R3 via Serial link (Subnet Address: 192.168.10.196 /30).

R3 is connected to R4 via Serial link (Subnet Address: 192.168.10.200 /30).

LAN network addresses

R1 network address: 192.168.10.0 /27. R2 network address: 192.168.10.32 /27.

R3 network address: 192.168.10.64 /27. R4 network address: 192.168.10.96 /27.



Now if you remember earlier we defined VLSM as subnetting a subnet. Below, the IP

address 192.168.10.0 (which is a subnet) will be broken down further; into smaller networks. Listed below are available subnets, and a list of Variable Length Subnets for

the mentioned network subnet.

Subnets of 192.168.10.0 (this can be termed as Subnet Number: 0 ) Subnet Address: 192.168.10 .0 /27

Subnet mask: 255.255.255.224

First Network Subnet Address: 192.168.10.32 /27 (if you look at the cram table, you will see that 3 bits were borrowed and this accommodates 30 usable host addresses, 2

is reserved for network and broadcast address.)

Second Network Subnet Address: 192.168.10.64 /27 (32 + 32 = 64)

Third Network Subnet Address: 192.168.10.96 /27 (64 + 32 = 96)

Four Network Subnet Address: 192.168.10.128 /27 (96 + 32 = 128)

Fifth Network Subnet Address: 192.168.10.160 /27 (128 + 32 = 160)

Sixth Subnet Network Address: 192.168.10.192 /27 (128 + 32 = 192)

As mentioned earlier, each network is has an equal host range of 30. In each block, we

reserved 2 for network and broadcast addressing.

Subnet for the WAN links

Subnets of 192.168.10.192 (subnet 0)

R1 – R2 Subnet Address: 192.168.10.192 /30 (192 + 4* = 196

R2 – R3 Subnet Address: 192.168.10.196 /30 (196 + 4 = 200)

R3 – R4 Subnet Address: 192.168.10.200 /30

*+ 4, because we need to minimize

wastage of ip address space. Two

usable address for each WAN link, one

for broadcast and one for network.



This is how the network IP addressing structure will look like:



Implementing VLSM #2.

Let’s look at another practice example, this time; we will work with different host ranges with different network masks for different router interfaces.

Scenario

Suppose as a network Administrator, you are presented with the following requirements

and network IP 192.168.10.0 /26: • London HQ = 58 host addresses

• Aberdeen HQ = 26 host addresses • Quebec HQ = 10 host addresses

• Idaho HQ = 10 host addresses • WAN links = 2 host addresses (each)

When implementing this VLSM subnetting scheme, we have to allow for some growth in

the number of hosts. This is the whole essence of VLSM Subnetting.

We will start from the largest to the smallest. According to the requirement, London, the largest requires 58 hosts.

If we follow the cram table, we will have to borrow 2 bits to use /26.

Bit Value 128 64 32 16 8 4 2 1

Bit Borrowed 1 2 3 4 5 6 7 8


Subnet Mask 128 192 224 240 248 252 255 256


192.168.10.0/26

– This will give us 62 usable host addresses reserving 2 for network and broadcast, with

the subnet mask of 192

London Network IP address: 192.168.10.0 Subnet mask: 255.255.255.192

Host Range: 192.168.10.1 to 192.168.10.62 Broadcast address: 192.168.10.63

(Next Network will start at 192.168.10.64)



Aberdeen (26 host address) We will use the next available network which is 192.168.10.64/27 (because of the number of host: 28)

Bit Value 128 64 32 16 8 4 2 1

Bit Borrowed 1 2 3 4 5 6 7 8


Subnet Mask 128 192 224 240 248 252 255 256


To minimize the waste of addresses, we are going to borrow 3 bits. If you look at the cram table, an address block of /27 will be sufficient: Aberdeen Network IP: 192.168.10.64/27 Subnet mask: 255.255.255.224 Host Range: 192.168.10.65 to 192.168.10.94 Broadcast address: 192.168.10.95 Next Network will start at 192.168.10.96 (That is 64 + 32 = 96 due to the network requirement of 10 hosts, and leaves some for expansion.) Quebec (10 usable host addresses)

Bit Value 128 64 32 16 8 4 2 1

Bit Borrowed 1 2 3 4 5 6 7 8


Subnet Mask 128 192 224 240 248 252 255 256


The 3 bits borrowed will accommodate our requirement at this time with different subnet mask. Quebec Network IP: 192.168.10.96 /28 Subnet mask: 255.255.255.240 Host range: 192.168.10.97 to 192.168.10.110 Broadcast address: 192.168.10.111

Next Network will start at 192.168.10.112 (That is 96 + 16 = 112 due to the network requirement of 10 hosts)



Idaho (10 usable addresses) Idaho network address will start at 192.168.10.112 /28 Subnet mask: 255.255.255.240 Host range: 192.168.10.113 to 192.168.10.126 Broadcast address: 192.168.10.127

Bit Value 128 64 32 16 8 4 2 1

Bit Borrowed 1 2 3 4 5 6 7 8


Subnet Mask 128 192 224 240 248 252 255 256


Next subnets will be for the WAN Links, which are point-to-point links. This require 2 addresses spaces each and will begin from: 192.168.10.128 We are borrowing 6 bits with bits value of 4 (according to host requirement) WAN 1 Network address: 192.168.10.128 /30 Host range: 192.168.10.129 to 192.168.10.130 /30 Subnet mask: 255.255.255.252 Broadcast: 192.168.10.131 Both routers will be assigned with an IP each and the remaining 2 addresses will be for network and broadcast. Next network will begin from 192.168.10.132 (128 + 4 =132) WAN 2 Network: 192.168.10.132 /30 Host range: 192.168.10.133 to 192.168.10.134 Subnet mask: 255.255.255.252 Broadcast: 192.168.10.135

Next network will begin from 192.168.10.136 (132 + 4 =136)



WAN 3 Network: 192.168.10.136 /30 Subnet mask: 255.255.255.252 Host range: 192.168.10.137 to 138 Broadcast: 192.168.10.139

Administrator’s Network IP Plan table

Name Subnet Address

Address Range

Broadcast Address

Network/ Mask prefix

London (58 hosts) 192.168.10.0 .1 - .62 .63 192.168.10.0/26

Aberdeen 192.168.10.64 .65 - .94 .95 192.168.10.64 /27

Quebec 192.168.10.96 .97 - .110 .111 192.168.10.96 /28

Idaho 192.168.10.112 .113 – 126 .127 192.168.10.112 /28

WAN 1- 2 192.168.10.128 .129 – 130 .131 192.168.10.128 /30

WAN 2 - 3 192.168.10.132 .133 – 134 .135 192.168.10.132 /30

WAN 3 - 4 192.168.10.136 .137 – 138 .139 192.168.10.136 /30



Refresher…

To really understand how VLSM works, you need to be

clear on how block sizes work to create a VLSM subnet

masks.

For example, when you need host requirement of 52, then

you will need the block size of 64, if you require 13 host,

you have to work with block size 16. Even if you need 45

hosts, block size of 64 will have to be your choice. You

don’t cut corners by trying to create or make up block

sizes! That’s why the cram table is so effective.

Bit Value + Bit Value = Subnet Mask

8 bits = 11111111 = 128+64+32+16+8+4+2+1 = 255 7 bits = 11111110 = 128+64+32+16+8+4+2= 254

6 bits = 11111100 = 128+64+32+16+8+4 = 252 5 bits = 11111000 = 128+64+32+16+8 = 248 4 bits = 11110000 = 128+64+32+16 = 240

3 bits = 11100000 = 128+64+32 = 224 2 bits = 11000000 = 128+64 = 192

1 bits = 10000000 = 128 = 128 0 bits = 00000000 = 0 = 0



VLSM Practice





VLSM Activity 1

Test your skills by filling in the appropriate values to complete the IP addressing structure

required.

IP Address 200.199.90.0

Try your VLSM Addressing Skill.

IP Address 172.17.10.0

Host

Requirement

/notation No. of hosts subnet Host Range Broadcast

24 /27 30 172.17.10.0 1 – 30 .31

24

25

10

6

2

Host

requirements

/slash No. of hosts Subnet Host Range Broadcast

60 /26 62 200.199.90.0 1 – 62 .63

28

25

12

2

2



CCNA Exam Essentials

Try and have put the cram table in your head, this will

help you to understand how IP addressing and subnetting works. Follow this procedure:

1. When you get to the exam testing Centre, write out

the cram table on the scribbler you will be given so that you can refer to it when facing subnetting question.

2. Determine your block size (128, 64, 32, 16, 8, 4,)

3. Don’t forget to reserve for the broadcast address.

(Number before the next subnet)

4. Don’t forget to copy and save config file and pinging to see if your configuration worked

Web Resources

http://www.tcpipguide.com/

http://www.9tut.com

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtm

http://www.subnetting-secrets.com/vlsm.html

http://www.techexams.net/forums/ccna-ccent/19607-vlsm-practice.html

http://www.certforums.com/forums/general-cisco-certifications/19828-vlsm-

examples.html



http://www.9tut.com/

http://www.9tut.com/



http://www.subnetting-secrets.com/vlsm.html

http://www.techexams.net/forums/ccna-ccent/19607-vlsm-practice.html

http://www.certforums.com/forums/general-cisco-certifications/19828-vlsm-examples.html

http://www.certforums.com/forums/general-cisco-certifications/19828-vlsm-examples.html

FreeVLSM eBook

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