VLSM Subnetting Explained with Examples

VLSM subnetting allows us to create subnets based on our requirements and network size. It allows us to create subnets with different subnet masks.

There are two types of subnetting: FLSM and VLSM. FLSM is easy but produces subnets having the same number of IP addresses. The same-sized subnets do not scale all networks. Let us take an example. We have an IP subnet having 12 IP addresses. We have two networks. The first network has seven hosts. The second network has three hosts.

If we use FLSM, we can create the following subnets.

Subnets Hosts in each subnet
2 6
3 4
4 3

We need seven hosts in the first subnet and three in the second subnet. None of the above-listed combinations fulfill our requirements. If we use VLSM, we can create two subnets. The first will provide eight IP addresses. The second will provide four IP addresses. This example shows how VLSM allows us to utilize IP addresses based on our requirements.

VLSM Subnetting example

The following image shows a network.

vlsm subnetting example network

The above network has the following requirements.

  • The development department needs 74 IP addresses.
  • The production department needs 52 IP addresses.
  • The administration department needs 28 IP addresses.
  • Three WAN links connect these departments.
  • Each WAN link needs two IP addresses.
  • The given default IP subnet is 192.168.1.0/24.

VLSM Subnetting step-by-step

In the first step, we calculate and arrange our IP requirements in descending order.

Each network needs two additional IP addresses, one for the network address and another for the broadcast address. All IP subnets reserve two IP addresses for these requirements. They use the first IP address for the network address and the last IP address for the broadcast address.

Since each network requires two additional IP addresses, we add two to the total host requirement of each network. After finalizing the host requirement, we find the block size that fulfills the host requirement.

A block size is the block of IP address we get when we convert a host bit into a network bit. The following table lists the block sizes we can have.

Host bit(s)12345678
Block size2 4 8 16 32 64 128 256
Host bits910111213141516
Block size512 1024 2048 4096 8192 16384 32768 65536
Host bits1718192021222324
Block size131072 262144 524288 1048576 2097152 4194304 8388608 16777216

The block size must be greater than or equal to the actual host requirement of the network.

Actual host requirement = Host requirement + Network address + broadcast address
Block Size >= Actual host requirement

The following table lists the block sizes that meet our requirements.

Segment Host requirement Actual requirement Block size
Production 52 54 64
Development 74 76 128
Administration 28 30 32
Wan link 1 2 4 4
Wan link 2 2 4 4
Wan link 3 2 4 4

In the next step, we arrange segments in descending order.

Segment Block size Descending order
Development 128 1
Production 64 2
Administration 32 3
Wan link 1 4 4
Wan link 2 4 5
Wan link 3 4 6

VLSM is an extended version of FLSM. It uses the same steps FLSM uses. In FLSM, we break the default subnet into the same-sized subnets. In VLSM, we further divide the created subnets into small-sized subnets based on our requirements in descending order.

vlsm subnetting example answer

In this example, first, we will perform FLSM from the development segment.

The development segment's block size is 128. The given IP subnet belongs to class C. In class C, the first 24 bits are reserved network bits. We cannot use the reserved network bits. We can use only host bits for subnetting. The default class C IP subnet has 6 (8 - 2 reserved host bits [30 and 31] ) host bits available for subnetting. Subnetting always goes from left to right without skipping a bit. Hence, the first host bit we can use in subnetting is the 25th. If we convert this host bit (25th) into a network bit, we get two subnets having 128 IP addresses each.

Subnet Subnet1 Subnet2
Network ID 192.168.1.0 192.168.1.128
First host address 192.168.1.1 192.168.1.129
Last host address 192.168.1.126 192.168.1.254
Broadcast ID 192.168.1.127 192.168.1.255

It fulfills the development section's requirements. We can assign the first subnet to the development section and use the second subnet for the next VLSM subnetting.

Segment Development
Requirement 74
CIDR /25
Subnet mask 255.255.255.128
Network ID 192.168.1.0
First hosts 192.168.1.1
Last hosts 192.168.1.126
Broadcast ID 192.168.1.127

Our next segment is the production department. Its block size is 64. If we convert the next host bit (26th), we get four subnets having 64 IP addresses each.

Subnet Subnet 1 Subnet 2 Subnet 3 Subnet 4
Network ID 0 64 128 192
First address 1 65 129 193
Last address 62 126 190 254
Broadcast ID 63 127 191 255

We cannot use the subnets 1 and 2. They contain the addresses we have already assigned to the development section. We can use the subnet 3 for the production section.

Segment Production
Requirement 52
CIDR /26
Subnet mask 255.255.255.192
Network ID 192.168.1.128
First hosts 192.168.1.129
Last hosts 192.168.1.190
Broadcast ID 192.168.1.191

Our next segment is the administration department. Its block size is 32. If we convert the next host bit (27th), we get eight subnets having 32 IP addresses each.

Subnet Sub 1 Sub 2 Sub 3 Sub 4 Sub 5 Sub 6 Sub 7 Sub 8
Net ID 0 32 64 96 128 160 192 224
First Host 1 33 65 95 129 161 193 225
Last Host 30 62 94 126 158 190 222 254
Broadcast ID 31 63 95 127 159 191 223 255

We cannot use the subnets 1, 2, 3, 4, 5, and 6. They contain the addresses we have already assigned to the development and production sections. We can use subnet 7 for the administration section.

Segment Administration
Requirement 28
CIDR /27
Subnet mask 255.255.255.224
Network ID 192.168.1.192
First hosts 192.168.1.193
Last hosts 192.168.1.222
Broadcast ID 192.168.1.223

Our remaining segments are WAN links. Their block size is 4. If we convert the next three host bits (28, 29, and 30th), we get sixty-four subnets having 4 IP addresses each.

0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 64, 68, 72, 76, 80, 84, 88, 92, 96, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, 144, 148, 152, 156, 160, 164, 168, 172, 176, 180, 184, 188, 192, 196, 200, 204, 208, 212, 216, 220, 224, 228, 232, 236, 240, 244, 248, 252, 256

We cannot use the subnets 1 to 56. They contain the addresses we have already assigned to the development, production, and administration sections. We can use subnets 57, 58, and 59 for the WAN links.

Subnet Subnet 57 Subnet 58 Subnet 59
Network ID 224 228 232
First host 225 229 233
Last host 226 230 234
Broadcast ID 227 231 235

Assign subnet 57 to the WAN link 1.

Subnet Subnet 57
Segments Wan Link 1
Requirement 2
CIDR /30
Subnet mask 255.255.255.252
Network ID 192.168.1.224
First hosts 192.168.1.225
Last hosts 192.168.1.226
Broadcast ID 192.168.1.227

Assign subnet 58 to the WAN link 2.

Subnet Subnet 58
Segments Wan Link 2
Requirement 2
CIDR /30
Subnet mask 255.255.255.252
Network ID 192.168.1.228
First hosts 192.168.1.229
Last hosts 192.168.1.230
Broadcast ID 192.168.1.231

Assign subnet 59 to the WAN link 3.

Subnet Subnet 59
Segments Wan Link 3
Requirement 2
CIDR /30
Subnet mask 255.255.255.252
Network ID 192.168.1.232
First hosts 192.168.1.233
Last hosts 192.168.1.234
Broadcast ID 192.168.1.235

The following image shows the final allocation of IP addresses.

flsm subnetting step by step

We have assigned IP addresses to all segments. The subnets 60, 61, 62, 63, and 64 are still available for further use.

ComputerNetworkingNotes CCNA Study Guide VLSM Subnetting Explained with Examples