1. VLSM and CIDR CSE 421 CCNA Exploration 2 Chapter 6
17/04/2017
VLSM and CIDR Review
CSE 421
CCNA Exploration 2
Chapter 6
Thanks to S Ward Abingdon and Witney College
S Ward Abingdon and Witney College

2. Topics Revision of classful and classless IP addressing
VLSM and CIDR
17/04/2017
Topics
Revision of classful and classless IP addressing
Revision of VLSM and benefits
Use of Classless Interdomain Routing (CIDR)
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3. Classful addressing VLSM and CIDR 17/04/2017
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4. Network part and host part
VLSM and CIDR
17/04/2017
Network part and host part
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5. Classful networks Address class First octet range Number of networks
VLSM and CIDR
17/04/2017
Classful networks
Address class
First octet range
Number of networks
Hosts per network
Class A
0 to 127
128 (less 0 and 127)
16,777,214
Class B
128 to 191
16,348
65,534
Class C
192 to 229
2,097,152
254
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6. IPv4 Address Exhaustion

7. Solutions
Long term:
Change to IP version 6. Plenty of addresses using a different scheme
Short term:
Use VLSM and CIDR to avoid wasting addresses
Use private addresses locally and NAT for internet access – lets many hosts share a few public addresses

8. Classful Subnetting
All subnets of a main network must have the same subnet mask.
This means that they must all have the same number of hosts.

9. Classful Subnetting 192.168.80.192/27 192.168.80.0/24 192.168.80.32/27
/27
/27
/27
/27
/27

10. Subnet 192.168.1.0/24 Need 6 networks, up to 26 hosts.
Borrow 3 bits, /27,
Gives 8 networks, up to 30 hosts.
Point to point need x3 = 84 wasted

11. Subnet 172.16.0.0/16 Need 6 networks, up to 500 hosts.
Borrow 7 bits, /23,
Gives 128 networks, up to 510 hosts.
Point to point need x3 = 1524 wasted

12. Waste Classful subnetting wastes addresses.
If you are using private addresses then you may not be bothered.
Waste of public addresses does matter.

13. Solution: Variable Length Subnet Masking (VLSM)
Create subnets as per specific host requirements.
Need 10 addresses
/28. Give them 16.
Need 500 addresses
/23. Give them 512.
CSE 421

14. Variable Length Subnet Masking (VLSM)
“If you know how to subnet, you can do VLSM.”
What’s the trick?
Always satisfy the requirements of your biggest LAN and then work your way down ….

15. Variable Length Subnet Masking (VLSM)

16. VLSM 201.1.0.0/22 Advertise? 201.1.4.0/23 Router A 201.1.6.0/24
How many entries does Router A has in it’s routing table?
/24

17. Route summarization 201.1.0.0/22 201.1.4.0/23 201.1.6.0/24
Octet 3 in binary
/22
/23
/24
/24
Same
Difference starts here
Same
Difference starts here
21 bits the same so use /21 for summary

18. Route summarization 201.1.0.0/22 Advertise 201.1.0.0/21 201.1.4.0/23
/24
A Supernet summarizes multiple network addresses with a mask that is less than (or a summary of) the classful mask.
/24

19. Route summarisation What address would summarise:
/16
/17
/17
15 the same altogether
/15
Octet 2 in binary
7 the same here
You can create static summary routes.
Dynamic routes can be summarised.

20. CIDR Concept
Classless InterDomain Routing (CIDR) introduced 1993 by IETF.
To CIDR-compliant routers, address class is meaningless.
The network portion of the address is determined by the network subnet mask, also known as the network prefix, or prefix length (/8, /19, etc.).

21. Classless Routing Protocols
Classless routing protocols are complaint with CIDR which means they understand
VLSM and
Route Summarization.
All routing protocols are classless nowadays.

22. VLSM Exercise Subnet 172.16.0.0/16 by borrowing 4 bits.
Then subnet the third subnet by borrowing 2 more bits.
Write out the subnet addresses and masks.

23. Subnetting /16
/20
/20
/20
/20
/20
/20
/20
/20
/20
/20
/20
/20
/20
/20
/20
/20
/22
/22
/22
/22

24. VLSM and CIDR
17/04/2017
The End
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