# VLSM Subnetting Examples and Calculation Explained

There are two types of subnetting: FLSM and VLSM. FSLM subnetting uses the same subnet mask for all subnets. VLSM subnetting allows us to choose subnet masks based on the network requirement.

VLSM extends FLSM. We first use FLSM to break the given subnet into the same-sized subnets. Then, we use FLSM again on created subnets to break them into smaller same-sized subnets. We repeat this process until we get the subnets that meet our requirements. In FLSM, we use subnetting charts. A subnetting chart tells us the number of subnets we can get from a given IP subnet having a specific number of host addresses.

### Class A Subnetting chart

 Subnet mask (In slash notation) Subnet mask (In decimal notation) Network bits Host bits Subnets Block Size or Total Hosts addresses Valid hosts addresses /8 255.0.0.0 0 24 1 16777216 16777214 /9 255.128.0.0 1 23 2 8388608 8388606 /10 255.192.0.0 2 22 4 4194304 4194302 /11 255.224.0.0 3 21 8 2097152 2097150 /12 255.240.0.0 4 20 16 1048576 1048574 /13 255.248.0.0 5 19 32 524288 524286 /14 255.252.0.0 6 18 64 262144 262142 /15 255.254.0.0 7 17 128 131072 131070 /16 255.255.0.0 8 16 256 65536 65534 /17 255.255.128.0 9 15 512 32768 32766 /18 255.255.192.0 10 14 1024 16384 16382 /19 255.255.224.0 11 13 2048 8192 8190 /20 255.255.240.0 12 12 4096 4096 4094 /21 255.255.248.0 13 11 8192 2048 2046 /22 255.255.252.0 14 10 16384 1024 1022 /23 255.255.254.0 15 9 32768 512 510 /24 255.255.255.0 16 8 65536 256 254 /25 255.255.255.128 17 7 131072 128 126 /26 255.255.255.192 18 6 262144 64 62 /27 255.255.255.224 19 5 524288 32 30 /28 255.255.255.240 20 4 1048576 16 14 /29 255.255.255.248 21 3 2097152 8 6 /30 255.255.255.252 22 2 4194304 4 2

### Class B Subnetting chart

 Subnet mask (In slash notation) Subnet mask (In decimal notation) Network bits Host bits Subnets Block Size or Total Hosts addresses Valid hosts addresses /16 255.255.0.0 0 16 1 65536 65534 /17 255.255.128.0 1 15 2 32768 32766 /18 255.255.192.0 2 14 4 16384 16382 /19 255.255.224.0 3 13 8 8192 8190 /20 255.255.240.0 4 12 16 4096 4094 /21 255.255.248.0 5 11 32 2048 2046 /22 255.255.252.0 6 10 64 1024 1022 /23 255.255.254.0 7 9 128 512 510 /24 255.255.255.0 8 8 256 256 254 /25 255.255.255.128 9 7 512 128 126 /26 255.255.255.192 10 6 1024 64 62 /27 255.255.255.224 11 5 2048 32 30 /28 255.255.255.240 12 4 4096 16 14 /29 255.255.255.248 13 3 8192 8 6 /30 255.255.255.252 14 2 16384 4 2

### Class C Subnetting chart

 Subnet mask (In slash notation) Subnet mask (In decimal notation) Network bits Host bits Subnets Block Size or Total Hosts addresses Valid hosts addresses /24 255.255.255.0 0 8 1 256 254 /25 255.255.255.128 1 7 2 128 126 /26 255.255.255.192 2 6 4 64 62 /27 255.255.255.224 3 5 8 32 30 /28 255.255.255.240 4 4 16 16 14 /29 255.255.255.248 5 3 32 8 6 /30 255.255.255.252 6 2 64 4 2

This tutorial is the tenth chapter of the tutorial series. Other chapters of this series are the following.

### VLSM Example

The following image shows an example network. We will perform VLSM subnetting for this network.

## VLSM Subnetting

In the first step, we arrange all segments in descending order based on their requirements.

Each segment needs two additional IP addresses: one for the network address and another for the broadcast address. Because of this, we must select a block size that provides more or equal IP addresses to the actual requirement for each segment.

```Actual requirement =  Total hosts + two (network and broadcast)
```
 No. Segment Host requirement Block size that fulfills the host IP requirement Valid hosts in block 1 LAN Segment1 29 32 30 (32 -2) 2 LAN Segment 2 21 32 30 (32 -2) 3 LAN Segment 3 12 16 14 (16-2) 4 LAN Segment 4 8 16 14 (16-2) 5 WAN Link 1 2 4 2 (4-2) 6 WAN Link 2 2 4 2 (4-2) 7 WAN Link 3 2 4 2 (4-2) 8 WAN Link 4 2 4 2 (4-2)

In the next step, we find the subnet mask in the slash notation that provides the required block size. Our first and second segments need a block size of 32. The subnet mask /27 provides a total of 8 subnets having 32 IP addresses each.

0-31, 32-63, 64-95, 96-127, 128-159, 160-191, 192-223, 224-255

We can assign the first two subnets to these segments.

0-31, 32-63, 64-95, 96-127, 128-159, 160-191, 192-223, 224-255

The next two segments need a block size of 16. The subnet mask /28 provides 16 subnets containing 16 IP addresses each.

0-15, 16-31, 32-47, 48-63,64-79, 80-95, 96-111, 112-127, 128-143, 144-159, 160-175, 176-191, 192-207, 208-223, 224-239, 240-255

We cannot use the first four subnets. They contain the addresses we have already assigned to the first and second segments. We can use the fifth and sixth subnets for the third and fourth LAN segments.

0-15, 16-31, 32-47, 48-63,64-79, 80-95, 96-111, 112-127, 128-143, 144-159, 160-175, 176-191, 192-207, 208-223, 224-239, 240-255

The next four segments need a block size of 4. The subnet mask /30 provides 64 subnets containing 4 IP addresses.

0-3, 4-7, 8-11, 12-15, 16-19, 20-23, 24-27, 28-31, 32-35, 36-39, 40-43, 44-47, 48-51, 52-55, 56-59, 60-63, 64-67, 68-71, 72-75, 76-79, 80-83, 84-87, 88-91, 92-95, 96-99, 100-103, 104-107, 108-111, 112-115, 116-119, 120-123, 124-127, 128-131, 132-135, 136-139, 140-143, 144-147, 148-151, 152-155, 156-159, 160-163, 164-167, 168-171, 172-175, 176-179, 180-183, 184-187, 188-191, 192-195, 196-199, 200-203, 204-207, 208-211, 212-215, 216-219, 220-223, 224-227, 228-231, 232-235, 236-239, 240-243, 244-247, 248-251, 252-255

The first 24 subnets contain the IP addresses we assigned in the previous segments. We can use the subnets 25, 26, 27, and 28 for these segments.

0-3, 4-7, 8-11, 12-15, 16-19, 20-23, 24-27, 28-31, 32-35, 36-39, 40-43, 44-47, 48-51, 52-55, 56-59, 60-63, 64-67, 68-71, 72-75, 76-79, 80-83, 84-87, 88-91, 92-95, 96-99, 100-103, 104-107, 108-111, 112-115, 116-119, 120-123, 124-127, 128-131, 132-135, 136-139, 140-143, 144-147, 148-151, 152-155, 156-159, 160-163, 164-167, 168-171, 172-175, 176-179, 180-183, 184-187, 188-191, 192-195, 196-199, 200-203, 204-207, 208-211, 212-215, 216-219, 220-223, 224-227, 228-231, 232-235, 236-239, 240-243, 244-247, 248-251, 252-255

The following image shows the assigned IP subnets to each segment.

The following table lists a summary of all subnets.

 Segment CIDR Subnet Mask Network Address Broad cast Address Valid host addresses LAN Segment1 /27 255.255.255.224 192.168.1.0 192.168.1.31 192.168.1.1 to 192.168.1.30 LAN Segment 2 /27 255.255.255.224 192.168.1.32 192.168.1.63 192.168.1.33 to 192.168.1.62 LAN Segment 3 /28 255.255.255.240 192.168.1.64 192.168.1.79 192.168.1.65 to 192.168.1.78 LAN Segment 4 /28 255.255.255.240 192.168.1.80 192.168.1.95 192.168.1.81 to 192.168.1.94 WAN Link 1 /30 255.255.255.252 192.168.1.96 192.168.1.99 192.168.1.97 to 192.168.1.98 WAN Link 2 /30 255.255.255.252 192.168.1.100 192.168.1.103 192.168.1.101 to 192.168.1.102 WAN Link 3 /30 255.255.255.252 192.168.1.104 192.168.1.107 192.168.1.105 to 192.168.1.106 WAN Link 4 /30 255.255.255.252 192.168.1.108 192.168.1.111 192.168.1.107 to 192.168.1.108

ComputerNetworkingNotes CCNA Study Guide VLSM Subnetting Examples and Calculation Explained