VLSM Documentation: Subnetting Essentials

1. From Allocation to Documentation

In the previous lesson, you created a plan that looked something like this:

• Subnet A: 192.168.10.0/25
• Subnet B: 192.168.10.128/26
• ...and so on.

To make this actionable, we need to expand it into a full documentation table with these key columns for each subnet:

• Network Address: The first address in the subnet. It cannot be assigned to a host.
• Prefix (or Subnet Mask): Defines the size of the subnet.
• First Usable IP: The first IP address you can assign to a device (e.g., a host or a switch SVI).
• Last Usable IP: The last IP address you can assign to a device.
• Broadcast Address: The last address in the subnet. It's used to send messages to all hosts in that specific subnet and cannot be assigned to a device.

2. The Calculation Process

The process for finding these values is simple and methodical. The key is using the boundary of the next subnet to define the end of the current one.

Here is the logic:

1. Start with the Network Address and Prefix from your allocation plan (e.g., 192.168.10.0/25).
2. Find the Next Network Address. You find this by adding the subnet's block size (or "increment") to the current network address. For a /25, the block size is 128, so the next network starts at 192.168.10.128.
3. Calculate the Broadcast Address. The broadcast address of your current subnet is simply the address right before the next network begins. So, 192.168.10.128 - 1 gives you 192.168.10.127.
4. Determine the Usable Range.
   • First Usable IP = Network Address + 1 (e.g., 192.168.10.1)
   • Last Usable IP = Broadcast Address - 1 (e.g., 192.168.10.126)

This method is foolproof, no matter how large or small the subnet is.

The following video provides an excellent walkthrough of this exact process. Pay close attention to how the instructor uses the start of the next network to work backward and find the broadcast address of the previous one.

3. The Final Document

Once you've done the calculations, you need to present them clearly. In a professional setting, this is often done in a spreadsheet which serves as your IP Address Management (IPAM) document. We'll formalize the IPAM structure in a later module, but for now, we'll focus on creating the core data table.

The following article shows what a completed documentation table looks like.

4. Guided Practice

Let's apply this to the practice problem from our last lesson. We were given the parent block 222.37.34.0/24 and created this allocation plan:

Your Task:
Complete the documentation for the first two subnets: Research & Dev and Management. Calculate their First Usable IP, Last Usable IP, and Broadcast Address.

5. Your Turn: Full Documentation Exercise

You asked for lots of practice, so here's a chance to put it all together.

Scenario: A company needs to set up its network across four locations and connect them. You are given the parent address block 192.168.10.0/24. The requirements are:

• Location B: 122 hosts
• Location D: 55 hosts
• Location A: 8 hosts
• Location C: 4 hosts
• Three point-to-point WAN links are needed to connect the locations (2 hosts each).

Your Task:
Create a complete VLSM documentation table.

1. First, create the basic allocation plan (like we did in the last lesson).
2. Then, fill out the full documentation table for all seven required subnets.

Your final table should have these columns: Subnet Name, Network Address, Prefix, First Usable IP, Last Usable IP, Broadcast Address.

Conclusion

Excellent work. You can now perform the entire VLSM process, from analyzing requirements to producing a complete, professional documentation table. This is the core skill of IP address planning.

Key Takeaways:

• The final output of a VLSM plan is a detailed table listing the Network Address, Prefix, Usable Range, and Broadcast Address for every subnet.
• The most reliable way to find a subnet's broadcast address is to find the starting address of the next subnet and subtract one.
• The usable range is simply one address after the network address and one address before the broadcast address.

In our next lesson, we will build directly on this. Now that you can create an efficient, documented plan, we will learn how to calculate its address utilization efficiency. This allows you to quantitatively prove how effective your VLSM design is and compare it to less efficient, fixed-size schemes.