What Is VLSM Subnetting Calculator

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.

VLSM

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)

VLSM
VLSM

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.

vlsm, subnetting, flsm, 2 routers, 3 routers,

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