# Introduction to Networking (Yarnfield)

Networking and Communications

Oct 24, 2013 (4 years and 6 months ago)

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Introduction to Networking
(
Yarnfield
)

Variable Length Subnet Masking (VLSM)

Objectives

Define VLSM

Describe the difference between
classful

subnetting

Describe the advantages of VLSM

Be able to perform VLSM operations on give IP addresses

Classful subnetting exercise

172.80.0.0 255.255.248.0

Find

The first five subnet addresses

First host, last host and broadcast of each subnet

Default gateway

How many subnets can be made?

How many hosts per subnet?

VLSM defined

More than one subnet mask

Using
classful

subnetting

Why?

172.80.8.0

172.80.8.1

15.254

172.80.40.0

172.80.40.1

47.254

172.80.32.0

172.80.32.1

39.254

172.80.24.0

172.80.24.1

31.254

172.80.16.0

172.80.16.1

23.254

We need

An IP address to perform VLSM on

The number of hosts involved in each part of the
network

We will...

Create a number of subnet masks that suit our needs
more efficiently than a
classful

subnetting

scheme could

Example using a Class C network address

120 hosts

60 hosts

30 hosts

192.168.1.0

Process

1.
Find the segment with the largest number of hosts
connected to it

2.
Find an appropriate subnet mask for the largest
segment

3.
Write down the subnet addresses to fit the subnet

4.
Take one of the newly created subnet addresses and
apply a new subnet mask to it that is more appropriate

5.
Write down the subnet addresses to fit the new subnet

6.
Repeat from step 4 for smaller segments

Example continued

1.
Find the segment with the largest number of hosts
connected to it

In the example the largest segment has 120 hosts
connected so we must start with this segment

To accomodate120 hosts we need to use 7 bits from
the host portion of the address (2
7

-

2 = 126)

Example continued

2.
Find an appropriate subnet mask for the largest
segment

If we have borrowed 7 bits for our hosts the subnet
mask (in binary) will be
11111111.1111111.1111111.1000000

Convert this to decimal and we get 255.255.255.128

Example continued

3.
Write down the subnet addresses to fit the subnet

Now we need to find the subnet addresses that this
subnet mask will create

256

128 = 128

Therefore the subnets would be 192.168.1.0 and
192.168.1.128 (remember we can now use subnet zero!)

We can now assign 192.168.1.0/25 to accommodate the
120 segment and have 192.168.1.128 to use for the other
two segments

120 hosts (126 in total)

192.168.1.0/25

60 hosts (62 in total)

30 hosts (30 in total)

Example continued

4.
Take one of the newly created subnet addresses and apply a
new subnet mask to it that is more appropriate

We still have two segments to deal with and we have a new
subnet address to work with of 192.168.1.128

We must start with the larger segment, which has 60 hosts

To accommodate 60 hosts we need to borrow 6 bits from the
host portion of the given IP address

2
6

2 = 62 hosts

This will give us a subnet mask of
1111111.1111111.1111111.11000000 which is the same as
255.255.255.192

Example continued

5.
Write down the subnet addresses to fit the new subnet

Now we need to find the subnet addresses that this
subnet mask will create

256

192 = 64

Therefore the new subnet addresses would be
192.168.1.128 and 192.168.1.192

We can now use 192.168.1.128/26 for the segment with
60 hosts

120 hosts (126 in total)

192.168.1.0/25

60 hosts (62 in total)

192.168.1.128/26

30 hosts (30 in total)

Example continued

4.
Take one of the newly created subnet addresses and
apply a new subnet mask to it that is more appropriate

We still have the segment with 30 hosts to deal with

We work this out in the same way as before

To accommodate 30 hosts we need to borrow 5 bits
from the host portion of the IP address

2
5

2 = 30 hosts

This will give us a subnet mask of
1111111.1111111.1111111.11100000 which is
255.255.255.224

Example continued

5.
Write down the subnet addresses to fit the new subnet

Now we need to find the subnet addresses that this
subnet mask will create

256

224 = 32

Therefore the new subnet addresses would be
192.168.1.192 and 192.168.1.224

We can now use 192.168.1.192/27 for the segment with
30 hosts

We still have the new 192.168.1.224 subnet which could
be used for future growth

Result

120 hosts (126 in total)

192.168.1.0/25

60 hosts (62 in total)

192.168.1.128/26

30 hosts (30 in total)

192.168.1.192/27

192.168.1.0

Summary

To determine the number of hosts a subnet can support
use the formula 2
n

2

Always start the process with the segment with the
largest amount of hosts to accommodate

Classless
subnetting

deals with the hosts as opposed to
classful

subnetting

which deals more with subnets

Exercise

192.168.2.0/24

7 remote sites, 30 hosts

each

P to P links

between routers

Remote A 30 hosts

Remote B 30 hosts

Remote C 30 hosts

Remote D 30 hosts

Remote E 30 hosts

Remote F 30 hosts

Remote G 30 hosts

Central

Exercise

192.168.3.0

Backbone

126 hosts

6 hosts

6 hosts

6 hosts

30 hosts

30 hosts

30 hosts

Questions...

...are there any?