Vlsm

1.0

172.16.1.0

1010100
10001111100
1011100101011100
101100011101001
1011110100011010
00001010010110010
1001010101100111
1111010101000101
1101001101010011
001010010101010
1010101000110010
010101001011000
110101100011010
11010100001011
001010100110
1001010010

VLSM

Variable-Length Subnet Mask

Workbook
Version 1.0
Version 1.0

192.168.10.0
192.168.10.96192.168.10.126

10.250.1.0

172.31.15.0

IP Address Classes
Class A

1 – 127

(Network 127 is reserved for loopback and internal testing)
00000000.00000000.00000000.00000000
Leading bit pattern
0
Network .
Host
.
Host
.
Host

Class B

128 – 191

Leading bit pattern

10

10000000.00000000.00000000.00000000

Class C

192 – 223

Leading bit pattern

11011000000.00000000.00000000.00000000

Class D

224 – 239

(Reserved for multicast)

Class E

240 – 255

(Reserved for experimental, used for research)

Network .

Network .

Network .

Network

Host

. Network

.

.

Host

Host

Private Address Space
Class A

10.0.0.0 to 10.255.255.255

Class B

172.16.0.0 to 172.31.255.255

Class C

192.168.0.0 to 192.168.255.255Default Subnet Masks
Class A

255.0.0.0

Class B

255.255.0.0

Class C

255.255.255.0

This workbook assumes you already have a background in subnetting. If you don’t you may
want to consider completing the IP Addressing and Subnetting Workbook.

Produced by: Robb Jones
jonesr@careertech.net
Frederick County Career & Technology Center
Cisco Networking Academy
Frederick CountyPublic Schools
Frederick, Maryland, USA
Special Thanks to Melvin Baker and Jim Dorsch
for taking the time to check this workbook for errors.
Inside Cover

What is VLSM
Variable Length Subnet Masks allow you a much tighter control over
your addressing scheme. If you use a class C address with a default subnet
mask you end up with one subnet containing 256 addresses. By using VLSM
you canadjust the number of subnets and number of addresses depending
on the specific needs of your network. The same rules apply to a class A or
B addresses.
VLSM is supported by Cisco, OSPF, Dual IS-IS, BGP-4, and EIGRP.
You need to configure your router for Variable Length Subnet Masking by
setting up one of these protocols. Then configure the subnet masks of the
various interfaces in the IPaddress interface sub-command. To use
supernet you must also configure IP classless routes.

The Box Method
The box method is the simplest way to visualize the breakdown of a
range of addresses into smaller different sized subnets.
Start with a square. The whole square
is a single subnet comprised of 256
addresses.

/24
255.255.255.0
256 Hosts
1 Subnet

0

255
1

Split the box inhalf and you get two
subnets with 128 addresses,

0

128

/25
255.255.255.128
128 Hosts
2 Subnets
Divide the box into quarters and you
get four subnets with 64 addresses,

255

127
0

128

63
/26
255.255.255.192
64 Hosts
4 Subnets
Split each individual square and you
get eight subnets with 32 addresses,

64

192

127
0

32

31
64

/27
255.255.255.224
32Hosts
8 Subnets
2

191

128

63
96

95

255
160

159
192

127

191
224

223

255

Split the boxes in half again and you
get sixteen subnets with sixteen
addresses,

0

128

32
15

16

47
48

/28
255.255.255.240
16 Hosts
16 Subnets
The next split gives you thirty two
subnets with eight addresses,

79

8

0

7

15

23
64

The lastsplit gives sixty four subnets
with four addresses each,
You can use these squares in any
combination to fit your addressing
needs.

79

87

0

8

7
16
19

64
68

96

79

83
84

183
224

191
232

215

223

136

247

160

255

168

135
143
167 175
144 152
176
184
147
155
179 187
148 156
180
188

63

151
192

159
200

183
224

191...