Academy Conference 2010
Frisco, Texas
Teaching VLSM
Barry Charter ~ North Arkansas College ~ Newly Retired 
Shawn Dennis ~ Arkansas State University Mt. Home Arkansas
CCNA CCAI
July 19, 2010
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Introduction
Contact Information
 Barry Charter
bc_charter@yahoo.com
and
 Shawn Dennis
sdennis@asumh.edu
© 2010 Cisco Systems, Inc. All rights reserved.
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Importance of understanding VLSM
 IPv4 address availability
 CCNA exam
 Further networking classes
 Networking careers
 What about IPv6?
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Note:
 INTRODUCE IP ADDRESSING AND BINARY THE
FIRST DAY!!
Do not wait until the chapter on IP Addressing & Subnetting
(D1 = Ch 5, E1 = Ch 6) to begin discussing the basics of IP
addressing and what a subnet mask is.
 Review IP Addressing and Binary daily
 Practice makes perfect!!
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Binary & IP Addressing
 Understanding binary is extremely important
 VLSM
 Wildcard masks
 IP Addresses
 32 bits
 Dotted decimal format – 4 octets
 Conversion for each octect
 Practice converting IP Addresses from binary to
decimal and vice versa.
 Introduce binary system to students early in first
course. I start the second day of class.
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Dividing Address Space
 The “Great Divide”
 Where the division occurs between host bits and network bits
 The “Magic Number”
 Decimal value of the bit to the immediate left of “Great Divide”
 Subnets will be in increments of the “Magic Number”
 Example
 Subnet Mask: /27 or 255.255.255.224
The “Great Divide”
The “Magic Number”
128
64
32
16
8
4
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1
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Subnetting vs. VLSM vs. CIDR
 Subnetting
 Dividing network address space into equal size subnets
 VLSM – Variable Length Subnet Mask
 Divide network address space into unequal size subnets
 Maximize use of IP address space
 CIDR – Classless Inter-domain Routing
 Subnet Mask specified using the prefix size
 255.255.255.0 = /24
 Use of VLSM requires use of routing protocols that support
CIDR
 RIPv2
 EIGRP
 OSPF
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Teaching Methods
Magic Box
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Magic Box Method
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What Length Subnet Mask Should You Use? –
Practice, Practice, Practice...
Number of Hosts
Required
# of Host Bits
Needed to
Support the
Required # of
Hosts
Subnet Mask
(CIDR Prefix)
Subnet Mask
(Dotted Decimal)
Number of Usable
Hosts Created
100
2
25
50
900
5
10
80
500
15
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What Length Subnet Mask Should You Use? –
Practice, Practice, Practice...
Number of Hosts
Required
# of Host Bits
Needed to
Support the
Required # of
Hosts
Subnet Mask
(CIDR Prefix)
Subnet Mask
(Dotted Decimal)
Number of Usable
Hosts Created
100
7 bits
/25
255.255.255.128
126
2
2 bits
/30
255.255.255.252
2
25
5 bits
/27
255.255.255.224
30
50
6 bits
/26
255.255.255.192
62
900
10 bits
/22
255.255.252.0
1022
5
3 bits
/29
255.255.255.248
6
10
4 bits
/28
255.255.255.240
14
80
7 bits
/25
255.255.255.128
126
500
9 bits
/23
255.255.254.0
510
15
5 bits
/27
255.255.255.224
30
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Determine Network Portion of Subnets
 Start with largest LAN
 Working down in LAN size to point-to-point links
 Determine number of bits needed to address the hosts
 Larger LANs will have smaller CIDR prefixes
 /30 prefix for point-to-point links
 Formula: 2n – 2
 n – number of bits
 2 unusable addresses
 Network address – first address in subnet
 Broadcast address – last address in subnet
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Teaching Method
Subnetting in a Box
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Subnetting in a Box Method
 Divide a piece of paper into smaller subnets
 Value in top left corner of each section – network
address
 Value in bottom right corner of each section –
broadcast address
 Last network address is the value of the appropriate
octet for subnet mask
 As bits are borrowed the network is divided into smaller
subnets.
 New subnet mask is indicated by first address of last
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Value in top left corner of each section – network address
Value in bottom right corner of each section – broadcast address
0
255
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Value in top left corner of each section – network address
Value in bottom right corner of each section – broadcast address
0
128
7 host bits /25
126 hosts
127
255
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Value in top left corner of each section – network address
Value in bottom right corner of each section – broadcast address
0
128
6 host bits /26
62 hosts
7 host bits /25
126 hosts
191
192
127
255
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Value in top left corner of each section – network address
Value in bottom right corner of each section – broadcast address
0
128
6 host
bits /26
62 hosts
7 host bits /25
126 hosts
191
192
224
4 host
bits /28
239
5 host
bits /27
240
30 hosts
248
243
251
247
255
244
127
223
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Here is an example VLSM Exercise 1
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Start with largest LAN
Working down in LAN size to point-to-point
links
Example:
list LANS in descending order
60 hosts
28 hosts
12 hosts
12 hosts
There are 3 WAN links
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Start with largest LAN
Working down in LAN size to point-to-point
links
Example:
Determine number of host bits needed
60 hosts – 6 host bits /26
28 hosts – 5 host bits /27
12 hosts – 4 host bits /28
12 hosts – 4 host bits /28
There are 3 WAN links – 2 host bits /30
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Start with the largest LAN
Working down in LAN size to point-to-point links
0
128
62 hosts
192.168.10.0 /26
63
64
60 hosts – 6 host bits /26
28 hosts – 5 host bits /27
12 hosts – 4 host bits /28
127
255
12 hosts – 4 host bits /28
There are 3 WAN links – 2 host bits /30
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Start with the largest LAN
Working down in LAN size to point-to-point links
0
128
62 hosts
192.168.10.0 /26
63
64
96
30
hosts
60 hosts – 6 host bits /26
192.168.10.64 /27
28 hosts – 5 host bits /27
12 hosts – 4 host bits /28
95
127
255
12 hosts – 4 host bits /28
There are 3 WAN links – 2 host bits /30
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Start with the largest LAN
Working down in LAN size to point-to-point links
0
128
62 hosts
192.168.10.0 /26
63
64
96
14 hosts
192.168.10.96 /28
30
hosts
60 hosts – 6 host bits /26
111
192.168.10.64 /27
112
14 hosts
28 hosts – 5 host bits /27
12 hosts – 4 host bits /28
192.168.10.112 /28
95
127
255
12 hosts – 4 host bits /28
There are 3 WAN links – 2 host bits /30
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Start with the largest LAN
Working down in LAN size to point-to-point links
0
128
136
131
132
160
139
140
62 hosts
192.168.10.0 /26
135
143
144
63
64
96
159
191
192
14 hosts
192.168.10.96 /28
30
hosts
60 hosts – 6 host bits /26
111
192.168.10.64 /27
112
14 hosts
28 hosts – 5 host bits /27
12 hosts – 4 host bits /28
192.168.10.112 /28
95
127
255
12 hosts – 4 host bits /28
There are 3 WAN links – 2 host bits /30
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Practice
Packet Tracer
Activities
Exercise 2
20 min
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Start with the largest LAN
Working down in LAN size to point-to-point links Ex 2
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Practice
Packet Tracer
Activities
Exercise 3
20 min
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Start with the largest LAN
Working down in LAN size to point-to-point links Ex 3
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Practice
Packet Tracer
Activities
Exercise 4
20 min
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Start with the largest LAN
Working down in LAN size to point-to-point links
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Teaching
Resources
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Cisco Learning Institute
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Other Resources
 Presentations
 http://www.cisco.com/web/learning/le31/le46/cln/clp/fastlane/Su
bnet_Calculator/index2.htm
 www.learntosubnet.com
 Subnet Calculators
 http://www.cisco.com/cgi-bin/Support/IpSubnet/home.pl
 www.solarwinds.com
 www.subnet-calculator.com/cidr.php
 http://www.subnetonline.com/pages/subnet-calculators/ipsubnet-calculator.php
 http://www.wildpackets.com/resources/free_utilities/ipsubnetcalc
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Questions?
Thank you for attending!
DON’T FORGET TO TURN IN EVALUATION 
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