Introduction to Networking (Yarnfield)

pigeoneggtrainsNetworking and Communications

Oct 24, 2013 (3 years and 11 months ago)

85 views

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

wastes IP addresses


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
mask

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
mask

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
mask


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
mask


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
mask


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?