VLSM

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This material was developed with funding from the National Science Foundation under Grant # DUE 1601612

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16 Hosts 14 Usable

128 Hosts 128 Usable Addresses

64 Hosts 62 Usable Addresses

Variable-Length Subnet Masking or (VLSM) is a better way to divide an IP network and is an improvement on Fixed Length Subnet Masking (FLSM). In FLSM, the same subnet mask is used for the entire subnet within a network, regardless of the number of hosts needed on any individual subnet. This is a wasteful approach to subnetting. The addresses not used in a subnet go to waste. VLSM is a way of "subnetting subnets," which means that VLSM allows you to divide an IP address space into more efficient subnets of different sizes. You can create subnets with very different host counts based on need without wasting large numbers of addresses.

FLSM

32 Hosts 30 Usable

Class C Network Subnetted

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Click each item for more information.

255.255.255.128

128 – 2 = 126

194.100.101.129 - 194.100.101.254

101 addresses

194.100.102.1 - 194.100.102.126

114 addresses

The disadvantage of using FLSM is that all subnets must be the exact same size even if the organization does not need large subnets. For example, ACME Corporation needs five different subnets. The largest subnet requires 100 addresses. With FLSM, all subnets share the same mask, so they are the same size. This means that the subnet requiring only 10 addresses wastes a lot of addresses. Click on each green box to see how many addresses each subnet wastes.

Range

116 addresses

194.100.100.1 - 194.100.100.126

128 Hosts 126 Usable Addresses

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Total wasted addresses

FLSM Example Using Class C Network

Required Subnets 100 Addresses (hosts) 50 Addresses 25 Addresses 12 Addresses 10 Addresses

Wasted Addresses

433

26 addresses

Click each subnet more information.

Subnet

Requires three full Class C networks

Class C Example = 194.100.100.0

Size (Hosts)

76 addresses

Mask

194.100.100.129 - 194.100.100.254

194.100.101.1 - 194.100.101.126

Bits

Required Subnets 100 Addresses (hosts) 50 Addresses 25 Addresses 12 Addresses 10 Addresses

16 Hosts 14 Usable

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16 – 2 = 14

Wasted Addresses

Start with largest network Work down to smallest networks

32 – 2 = 30

194.100.100.129 - 194.100.100.190

64 – 2 = 62

255.255.255.224

The advantage of using VLSM is that all subnets DO NOT have to be equal in size. Subnets will be a power of 2 (2, 4, 8, 16, 32, 64, 128, 256). Start with the largest network. Since subnet 1 requires 100 addresses, allocate a range size of 128 (with 126 usable hosts) to accommodate the needed 100 addresses. Remember that the lowest address is reserved for the network address and the highest address is reserved as the broadcast address. Subnet 1 will use 194.100.100.1-194.100.100.126 and the broadcast address is 194.100.100.127. Subnet 2 requires 50 addresses; use a range of 64 (with 62 usable hosts). Subnet 2 picks up where subnet 1 finished, so subnet 2 starts at 128 and goes to 191. The network takes the first address in the range, 192.100.100.128, the broadcast address is the last in the range, 192.100.100.191, and the subnet mask is 255.255.255.192. Subnet 3 requires 25 addresses, so a range of 32 addresses (with 30 usable hosts) will suffice. Subnet 3 starts at 192 (the network address), uses 193 to 222 as usable host addresses, and reserves 223 for the broadcast address. The subnet mask is 255.255.255.224. Subnet 4 only needs 12 addresses, so use a range size of 16 (with 14 usable hosts). Subnet 4 starts at 224 for the network address, allocates 225-238 for usable host addresses, and the 239 address is the broadcast. The subnet mask is 255.255.255.240. Subnet 5 requires 10 addresses. Using a range size of 16 means that subnet 5 starts at 240 for the network address and 241-254 for usable host addresses. The 255 address is reserved for the broadcast. The subnet mask for subnet 5 is also 255.255.255.240 With VLSM, all five subnets can fit in the space of one Class C network.

194.100.100.225 - 194.100.100.238

64 Hosts 62 Usable Addresses

255.255.255.192

194.100.100.193 - 194.100.100.222

12 addresses

255.255.255.240

194.100.100.241 - 194.100.100.254

VLSM Example Using Class C Networks

255.255.255.254

8 Hosts 6 Usable

255.255.255.252

252

240

192

24 = 16

Example VLSM Sizes Class C Network

27 = 128

25 = 32

#Subnets

255.255.255.191

254

248

224

128

21 = 2

22 = 4

Subnet Chart Made Easy

255

23 = 8

4 Hosts 2 Usable

255.255.255.248

26 = 64

Remember each VLSM subnet must start where the last subnet finished. The subnet's size must always be a power of 2. Click on each of the colored boxes to see the associated subnet sizes. The charts show the corresponding subnet mask calculation.

2 – 2 = 0

4 – 2 = 2

8 – 2 = 6

256

#Host (-2)

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Start with largest network. Work down to smallest networks

126

.224/28

.192/28

.240/28

.48/28

194.100.19.0/24

194.100.17.160/27

.112/28

VLSMs can be used to allocate subnets across multiple classful networks. In this example, the VLSM subnets are allocated across four Class C networks. Remember to start with the largest subnet and work down to the smallest.

194.100.17.192/27

194.100.17.224/27

.0/28

194.100.16.64/26

194.100.16.0/24

194.100.18.64/27

.144/28

194.100.18.0/27

.208/28

194.100.16.128/26

.128/28

.64/28

194.100.18.32/27

.16/28

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194.100.16.192/26

194.100.17.0/26

194.100.18.96/27

194.100.17.0/24

.160/28

Occupying Multiple Classful Networks

.80/28

194.100.17.64/26

194.100.16.0/26

194.100.17.128/27

194.100.18.0/24

.176/28

.32/28

.96/28

Super Nets

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Super Network 194.100.18.0/23 or 255.255.254.0 194.100.18.1 - 194.100.19.254 Broadcast 194.100.19.255

VLSM’s can also be used to combine two or more smaller subnets into one larger subnet. This is referred to as super netting. Each super net in the example uses two Class C networks to create each super network.

Super Network 194.100.16.0/23 or 255.255.254.0 194.100.16.1 - 194.100.17.254 Broadcast 194.100.17.255

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Here we are using four Class C networks to create one super net.

Super Net 194.100.16.0/22 or 255.255.252.0 Range 194.100.16.1 - 194.100.19.254 Broadcast 194.100.19.255

4,096

Broadcast Address =

Use the subnet mask to identify the magic number.

8,192

Calculate the network address for the subnet.

256 – 224 = 32

Magic Number

16,384

512

Calculate the broadcast address

= 210.20.21.64 Network Address

32,768

1,024

2,048

255.255.255.255

Ip Address = 210.20.21.74

255.255.240.255

Step 2

Step 2 Use the binary IP address and subnet mask to determine the network address.

255.255.224.0

Step 3

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Step 1

255.255.192.0

Step 1 To find the magic number, subtract the last number in the subnet mask from 256.

255.255.254.0

Default Subnet Mask = 255.255.255.224

255.255.128.0

210.20.21.74

Click to View Magic Number Table

255.255.252.0

Identify Network Information from Host

A common task is to identify network information from a host on a VLSM network. In the example, you have a host with an address 210.20.21.74 and a subnet mask of 255.255.255.224. You need to identify the network and broadcast addresses. Click each step to reveal the process used.

255.255.248.0

Step 3 Take the network address, 210.20.21.64, add the magic number, 32, and subtract one.

210.20.21.96 – 1 = 210.20.21.95

Subnet Mask

4 Hosts 2 Usable Addresses 255.255.255.252

8 Hosts 6 Usable Addresses 255.255.255.248

16 Hosts 14 Usable Addresses 255.255.255.240

128 Hosts 126 Usable Addresses 255.255.255.128

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VLSM Examples

32 Hosts 30 Usable Addresses 255.255.255.224

Use the network address space below to allocate the following subnets using VLSMs. Network = 200.1.10.0 and Network 200.1.11.0 Range 200.1.10.1 - 200.1.11.254

Click the correct subnet size for Subnet 1 to see the needed addresses.

Click Next to continue.

64 Hosts 62 Usable Addresses 255.255.255.192

128 Hosts 126 Usable Mask: 255.255.255.128 Network: 200.1.10.0 Range: 200.1.10.1 to 200.1.10.126 Broadcast: 200.1.10.127

Needed Addresses 100

256 Hosts 254 Usable Addresses 255.255.255.0

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Click the correct subnet size for Subnet 2 to see the needed addresses.

128 Hosts 126 Usable Mask: 255.255.255.128 Network: 200.1.10.128 Range: 200.1.10.129 to 200.1.10.254 Broadcast: 200.1.10.255

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Needed Addresses 50

Click the correct subnet size for Subnet 3 to see the needed addresses.

64 Hosts 62 Usable Mask: 255.255.255.192 Network: 200.1.11.0 Range: 200.1.11.1 to 200.1.11.62 Broadcast: 200.1.11.63

64 Hosts 62 Usable Mask: 255.255.255.192 Network: 200.1.11.64 Range: 200.1.11.65 to 200.1.11.126 Broadcast: 200.1.11.127

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Click the correct subnet size for Subnet 4 to see the needed addresses.

Click the correct subnet size for Subnet 5 to see the needed addresses.

Needed Addresses 20

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32 Hosts 30 Usable Mask: 255.255.255.224 Network: 200.1.11.128 Range: 200.1.11.129 to 200.1.11.158 Broadcast: 200.1.11.159

Needed Addresses 10

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Click the correct subnet size for Subnet 6 to see the needed addresses.

16 Hosts 14 Usable Mask: 255.255.255.240 Network: 200.1.11.160 Range: 200.1.11.161 to 200.1.11.174 Broadcast: 200.1.11.175

Click the correct subnet size for Subnet 7 to see the needed addresses.

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16 Hosts 14 Usable Mask: 255.255.255.240 Network: 200.1.11.176 Range: 200.1.11.177 to 200.1.11.190 Broadcast: 200.1.11.191

Needed Addresses 5

8 Hosts 6 Usable Mask: 255.255.255.248 Network: 200.1.11.192 Range: 200.1.11.193 to 200.1.11.198 Broadcast: 200.1.11.199

Click the correct subnet size for Subnet 8 to see the needed addresses.

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Required Subnets Subnet 1 Requires 100 Addresses Subnet 2 Requires 100 Addresses Subnet 3 Requires 50 Addresses Subnet 4 Requires 50 Addresses Subnet 5 Requires 20 Addresses Subnet 6 Requires 10 Addresses Subnet 7 Requires 10 Addresses Subnet 8 Requires 5 Addresses

5 32 Hosts 30 Usable

VLSM Example Summary

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The summary for VLSM example shows the subnet allocation required to meet the addressing needs.