Do you STILL suck at subnetting?? (THE FINAL TEST) // EP 9

TL;DR

VLSM allows different subnet sizes carved from a single larger network block.

Briefing Cornell Notes

Briefing

A single /24 network—172.21.42.0/24—can be carved into multiple smaller subnets of different sizes to meet varying host needs, as long as Variable Length Subnet Masking (VLSM) is used. The challenge is to create four networks (workers, robots, servers, and guests) from one starting block that normally supports only 253 usable hosts, then allocate address space efficiently so each subnet has enough room for its required number of hosts.

The method hinges on one practical rule: when using VLSM, start with the largest host requirement and work down. Workers are the biggest demand at 117 hosts. To support that many hosts, the subnet must provide at least seven host bits (since 2^7 − 2 = 126 usable addresses). That yields a /25 mask (255.255.255.128) with an increment of 128 in the last octet. The workers network becomes 172.21.42.0 through 172.21.42.127.

Next come the robots, requiring 57 hosts. Continuing from where the previous allocation ended, the next available block starts at 172.21.42.128. For 57 hosts, six host bits are sufficient (2^6 − 2 = 62 usable), which corresponds to a /26 mask (255.255.255.192). The increment is 64, so the robots subnet spans 172.21.42.128 through 172.21.42.191.

Servers then require 26 hosts. The next starting point is 172.21.42.192. For 26 hosts, five host bits work (2^5 − 2 = 30 usable), producing a /27 mask (255.255.255.224). That allocation covers 172.21.42.192 through 172.21.42.223.

Finally, guests need 14 hosts. The remaining block begins at 172.21.42.224, and a /28 mask (255.255.255.240) provides four host bits (2^4 − 2 = 14 usable). The guest subnet is 172.21.42.224 through 172.21.42.239.

The takeaway is that “subnetting a subnet” is possible—and different-sized subnets are not only allowed but expected in real network design. The process is essentially the same subnetting logic used throughout the series, but applied in a disciplined order: biggest-to-smallest host requirements, each time selecting the next available network boundary and choosing the smallest mask that still satisfies the host count. The result is a clean VLSM plan using masks of /25, /26, /27, and /28 carved out of the original /24 without wasting address space.

Cornell Notes

The core lesson is that one IPv4 /24 network (172.21.42.0/24) can be split into multiple subnets with different sizes using VLSM, matching each subnet’s mask to its host requirement. The workflow is to allocate from largest host need to smallest, always starting at the next available network boundary. Workers (117 hosts) get a /25 (255.255.255.128) covering 172.21.42.0–127. Robots (57 hosts) get a /26 covering 172.21.42.128–191, servers (26 hosts) get a /27 covering 172.21.42.192–223, and guests (14 hosts) get a /28 covering 172.21.42.224–239. This demonstrates efficient address planning and confirms that “different-size subnets” is the point of VLSM.

Why does VLSM require starting with the largest host requirement?

VLSM allocates address space in blocks whose size depends on the subnet mask. If a smaller subnet is allocated first, it can consume address ranges that a larger subnet would later need. Starting with the largest host requirement ensures the biggest block is reserved first, then the remaining space is divided into progressively smaller blocks. In this case, workers (117 hosts) are allocated first, then robots (57), then servers (26), then guests (14).

How do you decide the subnet mask for a given host requirement?

Choose the number of host bits so that 2^hostBits − 2 is at least the required usable hosts. For workers (117 hosts), seven host bits give 2^7 − 2 = 126 usable, so the mask is /25. For robots (57), six host bits give 2^6 − 2 = 62 usable, so the mask is /26. For servers (26), five host bits give 2^5 − 2 = 30 usable, so the mask is /27. For guests (14), four host bits give 2^4 − 2 = 14 usable, so the mask is /28.

What does “increment” mean in the subnetting steps, and how is it used here?

The increment is the step size between consecutive subnet network addresses in the last octet. It equals 256 − subnetMaskLastOctetValue (or equivalently 2^(number of host bits)). For /25, the increment is 128; for /26 it’s 64; for /27 it’s 32; for /28 it’s 16. Those increments determine the next network boundary: workers end at 127, so robots start at 128; robots end at 191, so servers start at 192; servers end at 223, so guests start at 224.

How do the subnet ranges come out from the chosen masks?

Once the mask and starting network address are set, the subnet range is determined by the increment. Workers: 172.21.42.0/25 spans 0–127. Robots: 172.21.42.128/26 spans 128–191. Servers: 172.21.42.192/27 spans 192–223. Guests: 172.21.42.224/28 spans 224–239. Each range includes the network address and broadcast address, while usable hosts are the interior addresses.

What does the final allocation prove about subnetting ‘a subnet’?

It proves that you can subdivide a larger network into smaller ones of different sizes without breaking the addressing rules, as long as each subnet is aligned to the correct network boundary and uses a mask sized for its host needs. Here, the original /24 is partitioned into /25, /26, /27, and /28 subnets that fit together cleanly and satisfy the host requirements.

Review Questions

Given a /24 network and host requirements of 100, 50, 20, and 10, what VLSM masks would you choose for each and in what order would you allocate them?
For a subnet mask of /27, what are the increment size and the usable host count?
Starting from 172.21.42.0/24, what is the next available network address after allocating a /26 subnet?

Key Points

1
VLSM allows different subnet sizes carved from a single larger network block.
2
Allocate subnets from the largest host requirement to the smallest to avoid wasting address space.
3
Pick the smallest subnet mask where 2^hostBits − 2 meets or exceeds the required usable hosts.
4
Use subnet “increments” to jump to the next network boundary (128 for /25, 64 for /26, 32 for /27, 16 for /28).
5
For 172.21.42.0/24, the final VLSM plan is: workers 172.21.42.0–127 (/25), robots 172.21.42.128–191 (/26), servers 172.21.42.192–223 (/27), guests 172.21.42.224–239 (/28).
6
The process is the same subnetting logic as standard subnetting, but applied iteratively with different masks.

Highlights

A single 172.21.42.0/24 can be split into four different-sized subnets using VLSM: /25, /26, /27, and /28.

Workers (117 hosts) require seven host bits, which maps to a /25 and the range 172.21.42.0–172.21.42.127.

Robots (57 hosts) fit into a /26 starting at 172.21.42.128, covering 172.21.42.128–172.21.42.191.

Guests (14 hosts) land in a /28 starting at 172.21.42.224, covering 172.21.42.224–172.21.42.239.

Topics

VLSM
Subnetting
Host Requirements
CIDR Masks
IPv4 Address Planning

Mentioned

VLSM
CCNA