VLSM – Variable Length Subnet Mask is also known as classless subnetting, subnet may have different numbers of hosts depending upon network requirement. It is a technology that divides a single network into many subnetworks with subnet masks of different lengths for different router interfaces.
In VLSM / VLSM Subnetting or classless subnetting, you have to do subnetting as per the network requirement.
| Classes | Default Subnet Mask | Default CIDR | Range (CIDR) |
| A | 255.0.0.0 | /8 | /8 – /15 |
| B | 255.255.0.0 | /16 | /16 – /23 |
| C | 255.255.255.0 | /24 | /24 – /30 |
VLSM Subnetting Calculator
How to find CIDR values, for class A
/8 11111111.00000000.00000000.00000000 255.0.0.0
/9 11111111.10000000.00000000.00000000 255.128.0.0
/10 11111111.11000000.00000000.00000000 255.192.0.0
/11 11111111.11100000.00000000.00000000 255.224.0.0
/12 11111111.11110000.00000000.00000000 255.240.0.0
/13 11111111.11111000.00000000.00000000 255.248.0.0
/14 11111111.11111100.00000000.00000000 255.252.0.0
15// 11111111.11111110.00000000.00000000 255.254.0.0
For class B
/16 11111111.11111111.00000000.00000000 255.255.0.0
/17 11111111.11111111.1000000.00000000 255.255.128.0
/18 11111111.11111111.11000000.00000000 255.255.192.0
/19 11111111.11111111.11100000.00000000 255.255.224.0
/20 11111111.11111111.11110000.00000000 255.255.240.0
/21 11111111.11111111.11111000.00000000 255.255.248.0
/22 11111111.11111111.11111100.00000000 255.255.252.0
/23 11111111.11111111.11111110.00000000 255.255.254.0
For class C
/24 11111111.11111111.11111111.00000000 255.255.255.0
/25 11111111.11111111.11111111.10000000 255.255.255.128
/26 11111111.11111111.11111111.11000000 255.255.255.192
/27 11111111.11111111.11111111.11100000 255.255.255.224
/28 11111111.11111111.11111111.11110000 255.255.255.240
/29 11111111.11111111.11111111.11111000 255.255.255.248
/30 11111111.11111111.11111111.11111100 255.255.255.252
We can’t use a /31 or /32 because we have atleast 2 host bit for assigning IP addresses to hosts.
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Example: two different networks are connected to each other. You have to find out the host of each network. 60 hosts are required for Router1 and 30 hosts are required for router2. Consider a network for Router1 192.168.0.0
Note: First Valid Host is the Default Gateway of the Router.
R1<=60 host
60 <=2n-2
60< =26-2
60<=64-2
60<=62
Now total number of network bits=32 (a network contain 32 bits)
=32 – n = 32-6 = 26
This 26 is your CIDR value of the network 192.168.0.0
That means 192.168.0.0/26
N.S.M 11111111.11111111.1111111.11000000
255.255.255.192
Block size= 256-192
= 64
192.168.0.0 Network ID
192.168.0.1 First Valid Host
192.168.0.62 Last Valid Host
192.168.0.63 Broadcast address
192.168.0.64 New Network(Router2)
R2<=30 hosts
30<=2n-2
30<=25-2
30<=32-2
30<=30
Now total number of network bits= 32
=32- n = 32-5 =27
This 27 is the CIDR value of network 192.168.0.64
That means 192.168.0.64/27
N.S.M 11111111.11111111.11111111.11100000
255.255.255.224
Block size = 256- N.S.M
= 256-224 = 32
192.168.0.64 Network ID
192.168.0.65 First Valid Host
192.168.0.94 Last Valid Host
192.168.0.95 Broadcast Address
192.168.0.96 New Network for R1 and R2
R1 & R2 <= 2 (we want only two hosts to connect both routers to the 3rd network)
3rd network is formed between two routers.
2<= 2n-2
2<=22-2
2<=4-2
2<=2
Now total number of network bits= 32
=32 – n = 32 – 2 = 30
This 30 is the CIDR value of network 192.168.0.96
That means 192.168.0.96/30
N.S.M 11111111.11111111.11111111.11111100
255.255.255.252
Block size = 256-252
= 4
192.168.0.96 Network ID
192.168.0.97 First Valid Host
192.168.0.98 Last Valid Host
192.168.0.99 Broadcast address
192.168.0.100 New Network (this network can be used for further connection)
What is an IP Address? Different classes, Range, Subnet Mask.
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Example 2: three different networks are connected to each other. You have to find out the host of each network. 3000 hosts required for Router1 and 2000 hosts required for router2 and 1000 hosts required for Router3. Consider a network for Router1 180.0.0.0.
R1<=3000
3000<=2n-2
3000<=212-2
3000<=4096-2
3000<=4094
Now total number of network bits = 32
=32 – n = 32 – 12 = 20
This 20 is the CIDR value of network 180.0.0.0
That means 180.0.0.0/20
N,S,M 11111111.11111111.11110000.00000000
255.255.240.0
Block size 256- N.S.M
256- 240 = 16
180.0.0.0 Network ID
180.0.0.1 First Valid Host
180.0.15.254 Last Valid Host
180.0.15.255 Broadcast address
180.0.16.0 New Network for Router 2
R2<= 2000
2000<=2n-2
2000<=211-2
2000<=2048-2
2000<=2046
Now total number of network bits = 32
= 32- n = 32 – 11 = 21
This 21 is the CIDR value of 180.0.0.16
That means 180.0.0.16/21
N.S.M 11111111.11111111.11111000.00000000
255.255.248.0
Block size 256- N.S.M
256-248 = 8
180.0.16.0 Network ID
180.0.16.1 First Valid Host
180.0.23.254 Last Valid Host
180.0.23.255 Broadcast address
180.0.24.0 New Network for Router 3
R3<=1000
1000<=2n-2
1000<=210-2
1000<=1024-2
1000<=1022
Now total number of network bits = 32
= 32- n = 32- 10 = 22
This 22 is the CIDR value of 180.0.0.24
That means 180.0.0.24/22
N.S.M 11111111.11111111.11111100.00000000
255.255.252.0
Block size 256-N,S,M
256-252 = 4
180.0.24.0 Network ID
180.0.24.1 Frist Valid Host
180.0.27.254 Last Valid Host
180.0.27.255 Broadcast address
180.0.28.0 New Network for Router 1 and Router 2
R1 & R2<=2
2<=2n-2
2<=22-2
2<=4-2
2<=2
Now total number of network bits = 32
= 32- n = 32 – 2 = 30
This 30 is the CIDR value of 180.0.28.0
That means 180.0.28.0/30
N.S.M 11111111.11111111.11111111.11111100
255.255.255.252
Block size 256- N.S.M
256- 252 = 4
180.0.28.0 Network ID
180.0.28.1 First Valid Host
180.0.28.2 Last Valid Host
180.0.28.3 Broadcast address
180.0.28.4 New Network for Router 2 and Router 3
R2 & R3 <=2
2<=2n-2
2<=22-2
2<=4-2
2<=2
Now total number of network bits = 32
= 32 – n = 32 – 2 = 30
This 30 is the CIDR value of network 18.0.28.4
That means 180.0.28.4/30
Block size 256- N.S.M
256- 252 = 4
180.0.28.4 Network ID
180.0.28.5 First Valid Host
180.0.28.6 Last Valid Host
180.0.28.7 Broadcast address
180.0.28.8 new network.
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