VLSM is a process of dividing an IP space into the subnets of different sizes without wasting IP addresses. When we perform subnetting, all subnets have the same number of hosts, this is known as FLSM ( Fixed length subnet mask). In FLSM all subnets use same subnet mask, this lead to inefficiencies. In real life scenario, some subnets may require large number of host addresses while other may require only few addresses.
For example, assume that you are a network administrator at Laxmisoftwares. Company have three departments connected with wan links.
- Development department have 74 computers.
- Production department have 52 computers.
- Administrative department have 28 computers.
- All departments are connected with each other via wan link.
- Each wan link requires two IP addresses.
With FLSM, to accumulate this requirement you have two choices, either purchase a class B IP address space or purchase at least two class C IP address space.
First choice with a example class B address space
172.168.1.0/23
Subnetting of this address space would give us 128 subnets and 512 hosts in each subnet. Our network requires only 6 subnets and 160 addresses. Every IP address add more dollars in company bill. You would have to pay for 65356 addresses while you need only 160 addresses. Would you consider this address space for company?
Second choice with two example class C address spaces
- 192.168.1.0/25
- 192.168.2.0/26
Subnetting of first address 192.168.1.0/25 would give us 2 subnets and 128 hosts in each subnet.
Subnetting of second address 192.168.2.0/26 would give us 4 subnets and 64 hosts in each subnet.
Collectively we are getting 6 subnets and 512 hosts from these two address spaces. We are still wasting more than 300 IP address, and we would have to purchase two address spaces.
Variable Length Subnet Mask
Variable Length Subnet Mask (VLSM) extends classic subnetting. VLSM is a process of breaking down subnets into the smaller subnets, according to the need of individual networks. In above example company have requirement of 6 subnets and 160 host addresses. With VSLM you can fulfill this requirement with single class C address space.
VLSM Subnetting
In VLSM Subnetting, we do subnetting of subnets according the network requirement.
Steps for VLSM Subnetting
- Find the largest segment. Segment which need largest number of hosts address.
- Do subnetting to fulfill the requirement of largest segment.
- Assign the appropriate subnet mask for the largest segment.
- For second largest segments, take one of these newly created subnets and apply a different, more appropriate, subnet mask to it.
- Assign the appropriate subnet mask for the second largest segment.
- Repeat this process until the last network.
VLSM Example
Now you know the steps of VLSM Subnetting. Let's understand it with above example. Our company requires 6 subnets and 160 hosts.
Step 1 :- Oder all segments according the hosts requirement (Largest to smallest).
| Subnet | Segment | Hosts |
| 1 | Development | 74 |
| 2 | Production | 52 |
| 3 | Administrative | 28 |
| 4 | Wan link 1 | 2 |
| 5 | Wan link 2 | 2 |
| 6 | Wan link 3 | 2 |
Step 2 :- Do subnetting for largest segment. Our largest segment needs 74 host addresses. /25 provide us two subnets with 128 hosts in each subnet.
192.168.1.0/25
| Subnet | Subnet 1 | Subnet 2 |
| 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 |
Step 3 :- Assign subnet mask to the largest segment. As you can see in above table, subnet 1 fulfill our largest segment requirement. Assign it to our segment.
| 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 |
Step 4 :- Do subnetting for second largest segment from next available subnet. Next segment requires 52 host addresses. Subnetting of /25 has given us two subnets with 128 hosts in each, from that we have assigned first subnet to development segment. Second segment is available, we would do subnetting of this.
/26 provide us 4 subnets with 64 hosts in each subnet.
192.168.1.0/26
| 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 subnet 1 and subnet 2 ( address from 0 to 127 ) as they are already assigned to development department. We can assign subnet 3 to our production department.
| 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 |
Step 5 :- Our next segment requires 28 hosts. From above subnetting we have subnet 3 and subnet 4 available. Do subnetting for the requirement of 28 hosts.
192.168.1.0/27
| 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 |
| LastHost | 30 | 62 | 94 | 126 | 158 | 190 | 222 | 254 |
| Broadcast ID | 31 | 63 | 95 | 127 | 159 | 191 | 223 | 255 |
Subnets 1 to 6 [ address from 0 to 191] are already occupied by previous segments. We can assign subnet 7 to this segment.
| Segment | Administrative |
| 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 |
Step 6 :- Our last three segments require 2 hosts per subnet. Do subnetting for these.
192.168.1.0/30
Valid subnets are:-
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
From these subnets, subnet 1 to subnet 56 ( Address from 0 - 220) are already assigned to previous segments. We can use 224,228, and 232 for 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 these subnets to wan links.
Wan Link 1
| 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 |
Wan Link 2
| 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 |
Wan link 3
| 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 |
We have assigned IP addresses to all segments, still we have 20 addresses available. This is the magic of VLSM.
Classful and classless, these two terms are also used for FLSM and VLSM.
Classful subnetting
FLSM is also known as classful subnetting as all subnets have same number of hosts. In classful subnetting all subnets use same subnet mask.
Classless subnetting
VLSM is also known as classless subnetting as all subnets may have different number of hosts depending upon network requirement.
Classful routing
RIPv1 and IGRP routing protocols do not have a field for subnet information. It means that if a router running RIP routing protocol, has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask. This is known as classful routing.
Classless routing
RIPv2, EIGRP and OSPF are known as classless routing protocols, as they have field for subnet information in their routing advertisement. VLSM only works with classless routing protocols.
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