VLSM Allocation: Ordering and Assigning Subnets

1. The Cardinal Rule: Largest to Smallest

The single most important rule in VLSM is to allocate subnets starting with the largest requirement and working your way down to the smallest.

Why is this so crucial? Imagine you have a large, empty plot of land (your parent address block). If you build a small shed (a small subnet) right in the middle, you might break up the remaining space into two smaller, unusable plots, preventing you from building a large warehouse (a large subnet) later. By allocating the largest plots first, you ensure that the remaining space is always a single, contiguous block, which can be further subdivided as needed. This prevents address space fragmentation.

The following short video clip clearly explains this first step.

2. The Allocation Process: A Walkthrough

Once your requirements are sorted, the allocation process is a methodical, step-by-step procedure of carving up your parent address block. You take the first (largest) requirement, assign it the first available subnet of the correct size, and then calculate where the next available address is.

The following video provides an excellent, detailed walkthrough of this process. It uses the same host-to-prefix chart we developed in the last lesson.

Let's break down the logic from the video:

1. Start with the parent block: 192.168.1.0/24.
2. Order the requirements: 100 hosts, 50 hosts, 10 hosts.
3. Allocate the largest subnet: The 100-host network needs a /25. The first available subnet is 192.168.1.0/25.
4. Find the next boundary: A /25 subnet contains 128 addresses. So, the first subnet uses addresses from 192.168.1.0 to 192.168.1.127. The next available address is 192.168.1.128.
5. Allocate the next subnet: The 50-host network needs a /26. We assign it to the next available block: 192.168.1.128/26.
6. Find the next boundary: A /26 subnet contains 64 addresses. Starting from .128, the next boundary is 128 + 64 = 192. The next available address is 192.168.1.192.
7. Continue the process: The 10-host network needs a /28, which is assigned to 192.168.1.192/28.

This iterative process of allocate -> find next boundary -> repeat is the essence of building a VLSM plan.

3. Subnetting Larger Blocks

Your goal is to design a network for ~1,000 devices, which means you'll be working with address blocks larger than a standard /24. The logic is identical, but the calculations will involve more than just the last octet.

Let's watch the second example from the same video, which demonstrates this with a /22 block.

As you saw, a /23 subnet for 500 hosts, starting at 10.10.0.0, has a block size of 2 in the third octet. This means it uses all addresses from 10.10.0.0 to 10.10.1.255. The next available network address is therefore 10.10.2.0. The process remains the same, you just need to be mindful of which octet your subnet mask affects.

4. Guided Practice

Let's apply this together. Imagine we're given the parent block 10.50.0.0/21 and the following requirements:

• Engineering LAN: 400 hosts
• Sales LAN: 150 hosts
• Servers: 80 hosts
• Management: 25 hosts
• WAN to HQ: 2 hosts

Step 1: Order requirements and determine prefixes (This combines last lesson's skill with this lesson's first rule).

Step 2: Allocate non-overlapping subnets.

1. Engineering (/23): Starts at the beginning of our parent block.

   • Network Address: 10.50.0.0/23
   • A /23 block uses 512 addresses. It spans 10.50.0.0 and 10.50.1.0.
   • Next available address is 10.50.2.0.

2. Sales (/24): Starts at the next available address.

   • Network Address: 10.50.2.0/24
   • A /24 block uses 256 addresses.
   • Next available address is 10.50.3.0.

3. Servers (/25):

   • Network Address: 10.50.3.0/25
   • A /25 block uses 128 addresses.
   • Next available address is 10.50.3.128.

4. Management (/27):

   • Network Address: 10.50.3.128/27
   • A /27 block uses 32 addresses. The next boundary is .128 + 32 = .160.
   • Next available address is 10.50.3.160.

5. WAN to HQ (/30):

   • Network Address: 10.50.3.160/30
   • A /30 block uses 4 addresses. The next boundary is .160 + 4 = .164.
   • Next available address is 10.50.3.164.

Our final allocation plan looks like this:

5. Your Turn: Independent Practice

You requested plenty of practice, so here is a problem to solve on your own. This will solidify the process.

Scenario: You are given the parent address block 222.37.34.0/24. You need to create a VLSM plan for the following requirements:

• Research and Development: 110 hosts
• Management: 60 hosts
• Marketing: 29 hosts
• Bookkeeping: 12 hosts
• Reception: 3 hosts

Your Task:

1. Order the requirements from largest to smallest.
2. Determine the optimal CIDR prefix for each requirement.
3. Develop the allocation plan by assigning a non-overlapping network address to each segment.

Present your final answer as a table with columns for the department, its assigned network address, and its CIDR prefix.

Conclusion

Fantastic work. You have now learned the complete, repeatable process for creating a VLSM allocation plan. This is a huge step toward designing the large, efficient network you're aiming for.

Key Takeaways:

• Always order your network requirements from largest to smallest before you begin allocating addresses.
• The allocation process is iterative: assign the first available block, calculate the next available address, and repeat.
• This process works the same way for large address blocks that span multiple octets; you just need to be careful with which octet you're incrementing.

In our next lesson, we'll take the plans we've created today and learn how to formally document them. This includes calculating and recording the full details for each subnet: its network address, prefix, usable host range, and broadcast address. This documentation is the final deliverable for any addressing plan.