Cisco

01.0
1010100 10001111100 1011100101011100 101100011101001 1011110100011010 00001010010110010 1001010101100111 1111010101000101 1101001101010011 001010010101010 1010101000110010 010101001011000 110101100011010 11010100001011 001010100110 1001010010

10.250.1.0

172.16.1.0

VLSM
Version 1.0 Version 1.0

Variable-Length Subnet Mask

Workbook

192.168.10.0 192.168.10.96192.168.10.126

172.31.15.0

IP Address Classes
Class A Class B Class C Class D Class E 1 – 127 128 – 191 192 – 223 224 – 239 240 – 255 (Network 127 is reserved for loopback and internal testing) 00000000.00000000.00000000.00000000 Leading bit pattern 0 Network . Host . Host . Host Leading bit pattern Leading bit pattern 10 110
10000000.00000000.00000000.00000000
Network . Network . Host . Host11000000.00000000.00000000.00000000
Network . Network . Network . Host

(Reserved for multicast) (Reserved for experimental, used for research)

Private Address Space
Class A Class B Class C 10.0.0.0 to 10.255.255.255 172.16.0.0 to 172.31.255.255 192.168.0.0 to 192.168.255.255

Default Subnet Masks
Class A Class B Class C 255.0.0.0 255.255.0.0 255.255.255.0

This workbook assumes youalready 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 County Public Schools Frederick, Maryland, USA Special Thanks to Melvin Baker and Jim Dorsch for taking the time to check this workbook forerrors. 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 can adjust 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 Baddresses. 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 IP address interface sub-command. To use supernet you must also configure IP classless routes.

The Box Method
The box method is the simplest way tovisualize 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.

0

/24 255.255.255.0 256 Hosts 1 Subnet

255
1

Split the box in half 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 foursubnets with 64 addresses,

127 0 128

255

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

191 192

64

127
0 32 128

255
160

31 64 96

63 192

159 224

191

/27 255.255.255.224 32 Hosts 8 Subnets
2

95

127

223

255

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

0 15 16 31 64

32 47 48 63 96 79 111 112 95 127
32 40

128 143 144 159 192 207 208 223
128 136

160 175 176 191 224 239 240 255
160 168

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

80

0

8

7 16 24

15 48

39 56

47

135 143 167 175 144 152 176 184

23 64 72

31 96

55 104

63

151 192159 200

183 224

191 232

/29 255.255.255.248 8 Hosts 32 Subnets
The last split gives sixty four subnets with four addresses each, You can use these squares in any combination to fit your addressing needs.

71 80 88

79

103 111 199 207 321 239 112 120 208 216 240 248

87

95

119

127

215

223

247

255

0 3 4 7 16 19 20 23 64

8 11 12 15 24 27 28 31 72 67 75...