1. Introduction to Networking
Variable Length Subnet Masking (VLSM)

2. What do you know?
Do you know how to convert decimal numbers to Base2 numbers and vice versa?
Do you know what a subnet is?
Do you know what VLSM stands for and what it is?
Do you know how to perform the VLSM process to devise a logical network scheme?
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Richard Hancock

3. Objectives
Be able to describe what subnetting is and what it’s benefits are
Be able to define VLSM and describe what it does
Be able to describe the advantages of VLSM
Be able to perform VLSM operations on given IP addresses
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Richard Hancock

4. Subnetting
The process of dividing a network up into subnets and to assign each subnet a valid network IP address, and the hosts on that subnet valid IP addresses
Subnet 1
Subnet 3
Subnet 2

5. Subnet benefits Makes larger networks more manageable
Reduces bandwidth consumption as a router must forward packets between subnets
Can provide a level of security in the network
But most importantly, it allows you to preserve your IP address allocation and use it more efficiently and effectively
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Richard Hancock

6. Types of subnetting There are two types of subnetting: Classful
Classless (VLSM)
Classful subnetting is used in older network protocols and has various issues that reduce it’s effectiveness
Classful subnetting would not allow you to use Subnet Zero
Classless subnetting (VLSM) is a more efficient system to preserve IP addresses and is used in modern routing protocols
Classless subnetting allows you to use Subnet Zero
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Richard Hancock

7. Some rules
You cannot use the Network address or the Broadcast address as a host address in either Classful or Classless subnetting!
Once a subnet address is allocated to a subnet with it’s subnet mask it cannot be used for subnetting again
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Richard Hancock

8. Variable Length Subnet Masks – – – – –
More than one subnet mask
Richard Hancock

9. We need An IP address to perform VLSM on
The number of segments we want to divide the major network into
The number of hosts involved in each part of each segment
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Richard Hancock

10. Example using a Class C network address
60 hosts
No subnet mask
120 hosts
No subnet mask
30 hosts
No subnet mask
Richard Hancock

11. Process
Find the segment with the largest number of hosts connected to it
Find an appropriate subnet mask for the largest segment
Write down the subnet addresses to fit the subnet mask
Take one of the newly created subnet addresses and apply a new subnet mask to it that is more appropriate
Write down the subnet addresses to fit the new subnet mask
Repeat from step 4 for smaller segments
Richard Hancock

12. Step 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
How many bits would we need to make 120?
To accomodate120 hosts we need to use 7 bits from the host portion of the address ( = 126)
120 hosts
60 hosts
30 hosts
Richard Hancock

13. Step 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
What is expressed in base10?
Converted to decimal (base10) we get
Richard Hancock

14. Step 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
Use the formula (256 - the subnet mask)
256 – 128 = 128
Therefore the subnets would be
and
We can now assign /25 to accommodate the 120 segment
can be used for further subnetting for the other two segments
Richard Hancock

15. So far... 60 hosts (62 in total) No subnet mask
/25
60 hosts (62 in total)
No subnet mask
30 hosts (30 in total)
Richard Hancock

16. Step 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
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
26 – 2 = 62 hosts
This will give us a subnet mask of
Converted to decimal this will be?
Done
60 hosts
(62 in total)
30 hosts
(30 in total)
Richard Hancock

17. Step 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
and
We can now use /26 for the segment with 60 hosts
We have left over to further subnet
Richard Hancock

18. So far... 60 hosts (62 in total) 192.168.1.128/26
/25
60 hosts (62 in total)
/26
30 hosts (30 in total)
No subnet mask
Richard Hancock

19. Step 4 is repeated
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
25 – 2 = 30 hosts
This will give us a subnet mask of which is
Richard Hancock

20. Step 5 is repeated
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 and
We can now use /27 for the segment with 30 hosts
We still have the new subnet which could be used for future growth
Richard Hancock

21. Result! 192.168.1.0 60 hosts (62 in total) 192.168.1.128/26
/25
60 hosts (62 in total)
/26
30 hosts (30 in total)
/27
Richard Hancock

22. Exercise 1 192.168.2.0/24 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
Central
Remote hosts
25 – 2 =30
( )
256 – 224 = 32
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27
Eight subnets created. First seven give to remote sites, eight subnet re-subnetted to accommodate the P to P links.
P to P links
22 – 2 = 2
( )
256 – 252 = 4
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (expansion)
Eight subnets created supporting 2 IP addresses. Only seven subnets are needed, leaving one left over for expansion.
Remote E 30 hosts
Remote F 30 hosts
Remote G 30 hosts
Richard Hancock

23. Exercise 1 solution Remote hosts 25 – 2 =30 hosts
( )
256 – 224 = 32
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27 (left for further subnetting)
Eight subnets created.
First seven give to remote sites; eighth subnet re-subnetted to accommodate the P to P links.
Richard Hancock

24. Exercise 1 solution continued
P to P links
22 – 2 = 2 hosts
( )
256 – 252 = 4
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (assigned to segment)
/30 (expansion)
Eight subnets created supporting 2 IP addresses
Only seven subnets are needed, leaving one left over for expansion.
Richard Hancock

25. Exercise 192.168.3.0 30 hosts 6 hosts Backbone 126 hosts 6 hosts
27 – 2 = 126
( )
256 – 128 = 128
/25(assigned to backbone)
/25
30 Hosts
25 – 2 = 30
( )
256 – 224 = 32
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27
6 hosts
23 – 2 =6
( )
256 – 248 = 8
/29 (assigned to segment)
/29 (assigned to segment)
/29 (assigned to segment)
/29 (expansion)
6 hosts
30 hosts
Richard Hancock

26. Exercise 2 solution Backbone 27 – 2 = 126
( )
256 – 128 = 128
/25(assigned to backbone)
/25
30 Hosts
25 – 2 = 30
( )
256 – 224 = 32
/27 (assigned to segment)
/27 (assigned to segment)
/27 (assigned to segment)
/27
6 hosts
23 – 2 =6
( )
256 – 248 = 8
/29 (assigned to segment)
/29 (assigned to segment)
/29 (assigned to segment)
/29 (expansion)
Richard Hancock

27. Summary
Classless subnetting (VLSM) is used in most networks and uses modern routing protocols
Subnetting is all about
Preserving IP addresses
Making large networks more manageable (logically)
Preserving bandwidth
Providing a level of security
To determine the number of hosts a subnet can support use the formula 2n – 2
Always start the VLSM process with the segment with the largest amount of hosts to accommodate
You cannot use the subnet address or broadcast address as a host address
Richard Hancock

28. Questions...
...are there any?
Richard Hancock

29. End!
Richard Hancock