VLSM – Review – Example

By adminRouting and Switching, Subnetting

VLSM table

Major Network = 10.15.0.0/22

11111111.11111111.11111100.00000000

Number of networks = 2^ 6 = 64

Network AddressUsable Host RangeBroadcast Address:
10.15.0.010.15.0.1 – 10.15.3.25410.15.3.255
10.15.4.010.15.4.1 – 10.15.7.25410.15.7.255
10.15.8.010.15.8.1 – 10.15.11.25410.15.11.255
10.15.12.010.15.12.1 – 10.15.15.25410.15.15.255
10.15.16.010.15.16.1 – 10.15.19.25410.15.19.255
10.15.20.010.15.20.1 – 10.15.23.25410.15.23.255
10.15.24.010.15.24.1 – 10.15.27.25410.15.27.255
10.15.28.010.15.28.1 – 10.15.31.25410.15.31.255
10.15.32.010.15.32.1 – 10.15.35.25410.15.35.255
10.15.36.010.15.36.1 – 10.15.39.25410.15.39.255
10.15.40.010.15.40.1 – 10.15.43.25410.15.43.255
10.15.44.010.15.44.1 – 10.15.47.25410.15.47.255
10.15.48.010.15.48.1 – 10.15.51.25410.15.51.255
10.15.52.010.15.52.1 – 10.15.55.25410.15.55.255
10.15.56.010.15.56.1 – 10.15.59.25410.15.59.255
10.15.60.010.15.60.1 – 10.15.63.25410.15.63.255
10.15.64.010.15.64.1 – 10.15.67.25410.15.67.255
10.15.68.010.15.68.1 – 10.15.71.25410.15.71.255
10.15.72.010.15.72.1 – 10.15.75.25410.15.75.255
10.15.76.010.15.76.1 – 10.15.79.25410.15.79.255
10.15.80.010.15.80.1 – 10.15.83.25410.15.83.255
10.15.84.010.15.84.1 – 10.15.87.25410.15.87.255
10.15.88.010.15.88.1 – 10.15.91.25410.15.91.255
10.15.92.010.15.92.1 – 10.15.95.25410.15.95.255
10.15.96.010.15.96.1 – 10.15.99.25410.15.99.255
10.15.100.010.15.100.1 – 10.15.103.25410.15.103.255
10.15.104.010.15.104.1 – 10.15.107.25410.15.107.255
10.15.108.010.15.108.1 – 10.15.111.25410.15.111.255
10.15.112.010.15.112.1 – 10.15.115.25410.15.115.255
10.15.116.010.15.116.1 – 10.15.119.25410.15.119.255
10.15.120.010.15.120.1 – 10.15.123.25410.15.123.255
10.15.124.010.15.124.1 – 10.15.127.25410.15.127.255
10.15.128.010.15.128.1 – 10.15.131.25410.15.131.255
10.15.132.010.15.132.1 – 10.15.135.25410.15.135.255
10.15.136.010.15.136.1 – 10.15.139.25410.15.139.255
10.15.140.010.15.140.1 – 10.15.143.25410.15.143.255
10.15.144.010.15.144.1 – 10.15.147.25410.15.147.255
10.15.148.010.15.148.1 – 10.15.151.25410.15.151.255
10.15.152.010.15.152.1 – 10.15.155.25410.15.155.255
10.15.156.010.15.156.1 – 10.15.159.25410.15.159.255
10.15.160.010.15.160.1 – 10.15.163.25410.15.163.255
10.15.164.010.15.164.1 – 10.15.167.25410.15.167.255
10.15.168.010.15.168.1 – 10.15.171.25410.15.171.255
10.15.172.010.15.172.1 – 10.15.175.25410.15.175.255
10.15.176.010.15.176.1 – 10.15.179.25410.15.179.255
10.15.180.010.15.180.1 – 10.15.183.25410.15.183.255
10.15.184.010.15.184.1 – 10.15.187.25410.15.187.255
10.15.188.010.15.188.1 – 10.15.191.25410.15.191.255
10.15.192.010.15.192.1 – 10.15.195.25410.15.195.255
10.15.196.010.15.196.1 – 10.15.199.25410.15.199.255
10.15.200.010.15.200.1 – 10.15.203.25410.15.203.255
10.15.204.010.15.204.1 – 10.15.207.25410.15.207.255
10.15.208.010.15.208.1 – 10.15.211.25410.15.211.255
10.15.212.010.15.212.1 – 10.15.215.25410.15.215.255
10.15.216.010.15.216.1 – 10.15.219.25410.15.219.255
10.15.220.010.15.220.1 – 10.15.223.25410.15.223.255
10.15.224.010.15.224.1 – 10.15.227.25410.15.227.255
10.15.228.010.15.228.1 – 10.15.231.25410.15.231.255
10.15.232.010.15.232.1 – 10.15.235.25410.15.235.255
10.15.236.010.15.236.1 – 10.15.239.25410.15.239.255
10.15.240.010.15.240.1 – 10.15.243.25410.15.243.255
10.15.244.010.15.244.1 – 10.15.247.25410.15.247.255
10.15.248.010.15.248.1 – 10.15.251.25410.15.251.255
10.15.252.010.15.252.1 – 10.15.255.25410.15.255.255

Number of hosts per network = 2^10 – 2 = 1022

Requirements:

Required # of HostsTotal # of Hosts based on host bits derivedUse
500 hosts 510Guest
200 hosts254Sales
100 hosts126Engineering
50 hosts62Finance
15 hosts30Future use
15 hosts30Future use
The Total # of hosts required = 880The total # of hosts allotted = 1,012

Total number of networks:

Using the following info, obtain the host bits and subnets

2^9512/2311111111.11111111.11111110.00000000255.255.254.0
2^8256/2411111111.11111111.11111111.00000000255.255.255.0
2^7128/2511111111.11111111.11111111.10000000255.255.255.128
2^664/2611111111.11111111.11111111.11000000255.255.255.192

Start with the first network based on the bitmask > note that the subnet mask will change.

The original IP block of 10.15.0.0/22 will produce 1022 usable hosts. The largest host requirement is 500 hosts, which equates to a /23. The original/22 can be broken down into two /23 subnets, which will produce 510 hosts each.

An increment of 2 is derived from the following bit mask where the “last” 1 bit is in the “2” column.

1286432168421
11111110

1st network assignment

Start with the given IP range of 10.15.0.0 and increment the third octet by two, which will produce 10.15.2.0.

The second network begins with 10.15.2.0/23, which means that you will subtract one from the third octet to produce the “end” IP range of the first network. Remember that the original network was divided into two.

NetworkBitmaskIncIst IPLast IPBroadcast AddressSM
10.15.0.0/2311111111.11111111.11111110.00000000210.15.0.110.15.1.25510.15.1.255255.255.254.0
10.15.2.0/2311111111.11111111.11111110.000000002N/AN/AN/AN/A

2nd network assignment

The second requirement is 200 hosts. The second subnet begins with 10.15.2.0/23, which will produce 510 hosts. The network can be further subdivided into two /24 networks, which will increment by 1 based on the bitmask as outlined in the table below.

NetworkBitmaskIncIst IPLast IPBroadcast AddressSM
10.15.2.0/2311111111.11111111.11111110.00000000N/AN/AN/AN/AN/A
10.15.2.0/2411111111.11111111.11111111.00000000110.15.2.110.15.2.25410.15.2.255255.255.255.0
10.15.3.0/2411111111.11111111.11111111.000000001UnusedUnusedUnusedUnused

The second network begins with 10.15.2.0/24, which means that you will subtract one from the third octet to produce the “end” IP range of the second network.

3rd network assignment

The third requirement is 100 hosts. The third subnet begins with 10.15.3.0/24, which will produce 256 hosts. The network can be further subdivided into two /25 networks, which will increment by 128 based on the bitmask as outlined in the table below.

NetworkBitmaskIncIst IPLast IPBroadcast AddressSM
10.15.3.0/2411111111.11111111.11111111.00000000N/AN/AN/A10.15.2.255N/A
10.15.3.0/2511111111.11111111.11111111.1000000012810.15.3.110.15.3.12610.15.3.127255.255.255.128
10.15.3.128/2511111111.11111111.11111111.10000000128UnusedUnusedUnusedUnused

The third network begins with 10.15.3.0/25, which means that you will subtract one from the fourth octet to produce the “end” IP range of the third network.

4th network assignment

The fourth requirement is 50 hosts. The third subnet begins with 10.15.3128/25, which will produce 128 hosts. The network can be further subdivided into two /26 networks, which will increment by 64 based on the bitmask as outlined in the table below.

NetworkBitmaskIncIst IPLast IPBroadcast AddressSM
10.15.3.128/2511111111.11111111.11111111.10000000NANANANANA
10.15.3.128/2611111111.11111111.11111111.110000006410.15.3.12910.15.3.19010.15.3.191255.255.255.192
10.15.3.192/2611111111.11111111.11111111.1100000064UnusedUnusedUnusedUnused

The remaining networks for future use

The remaining subnet(s) will begin at 10.15.3.192/26, which can be incremented by “64”, which will produce a subnet with 62 hosts, or it can be broken down one final time to a /27, which will produce 30 hosts each.

NetworkBitmaskIncIst IPLast IPBroadcast AddressSM
10.15.3.192/2611111111.11111111.11111111.110000006410.15.3.19210.15.3.25410.15.3.255255.255.255.192

OR

NetworkBitmaskIncIst IPLast IPBroadcast AddressSM
10.15.3.192/2711111111.11111111.11111111.111000003210.15.3.19310.15.3.22210.15.3.223255.255.255.224
10.15.3.224/2711111111.11111111.11111111.111000003210.15.3.22510.15.3.25410.15.3.255255.255.255.224

Completed network assignments

Subnet NameNeeded SizeAllocated SizeAddressMaskDec MaskAssignable RangeBroadcast
Guest50051010.15.0.0/23255.255.254.010.15.0.1 – 10.15.1.25410.15.1.255
Saes20025410.15.2.0/24255.255.255.010.15.2.1 – 10.15.2.25410.15.2.255
Engineering10012610.15.3.0/25255.255.255.12810.15.3.1 – 10.15.3.12610.15.3.127
Finance506210.15.3.128/26255.255.255.19210.15.3.129 – 10.15.3.19010.15.3.191
Future use153010.15.3.192/27255.255.255.22410.15.3.193 – 10.15.3.22210.15.3.223
Future use153010.15.3.224/27255.255.255.22410.15.3.225 – 10.15.3.25410.15.3.255

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