Mapping VLANs to VLSM in IPAM

The Foundation: The VLSM Allocation Process

Before we touch an IP address, we need a clear, repeatable process. You've already learned the mechanics of VLSM, but applying it to a large plan requires discipline. The core idea is to allocate the largest subnets first to avoid fragmenting your address space.

Let's review the step-by-step process for VLSM allocation.

This process of "sort, allocate, repeat" is exactly what we will use to populate our IPAM document.

Scenario: The AlphaCorp Network Plan

Let's work with a practical scenario. You are designing the network for a company called "AlphaCorp," which has two buildings, HQ and Engineering. You have been given the parent address block 10.50.0.0/16 to use for the entire company.

From the previous module's work, you have the following VLAN plan based on the company's requirements, including a 25% growth forecast:

Our task is to take this plan and create a detailed IPAM entry for each requirement.

Step-by-Step IPAM Population

We will now execute the VLSM process, populating our IPAM document as we go. We start with a blank version of the IPAM sheet we designed in the last lesson. Our parent block is 10.50.0.0/16.

1. Sort Requirements

First, we sort the list by the "Future Hosts" column, from largest to smallest.

1. HQ Corporate Users: 438 hosts
2. Guest WiFi: 250 hosts
3. Engineering Users: 225 hosts
4. Voice: 113 hosts
5. HQ WiFi: 100 hosts
6. Engineering WiFi: 57 hosts
7. Servers: 50 hosts
8. Network Management: 19 hosts
9. P2P Link: 2 hosts

2. Allocate and Document

Now, we go down the sorted list, one by one.

Requirement 1: HQ Corporate Users (438 hosts)

• Calculation: We need 438 usable addresses. A /24 gives 254, which is too small. A /23 gives 510, which is perfect.
• Allocation: We take the first available block from our parent range: 10.50.0.0/23.
• Documentation: We create the first row in our IPAM document. We'll assign the gateway (SVI) to be the first usable IP, .1, a common practice.

IPAM Document (Version 1):

Requirement 2: Guest WiFi (250 hosts)

• Calculation: 250 hosts. A /24 gives 254 addresses. This is a tight fit, but acceptable. To allow for more growth, a /23 (510 hosts) might be better, but we will stick to the most efficient size for this exercise, /24.
• Allocation: Our last subnet ended at 10.50.1.255. The next available address is 10.50.2.0. We allocate 10.50.2.0/24.
• Documentation: We add the second row. Note the different VRF.

IPAM Document (Version 2):

Requirement 3: Engineering Users (225 hosts)

• Calculation: 225 hosts. A /24 (254 usable) is the right fit.
• Allocation: The last subnet ended at 10.50.2.255. The next available address is 10.50.3.0. We allocate 10.50.3.0/24.
• Documentation: We add the third row.

IPAM Document (Version 3):

We continue this process for every requirement until our sorted list is complete. For the smaller subnets:

• Servers (50 hosts): Needs a /26 (62 usable IPs).
• Network Management (19 hosts): Needs a /27 (30 usable IPs).
• P2P Link (2 hosts): Needs a /30 (2 usable IPs).

The key is to always find the end of the last allocated subnet and start the new one at the very next address.

This process gives you a complete, non-overlapping address plan documented in one place. The Cisco guide below contains a fantastic example of a finished plan that looks very similar to what we are building.

From Spreadsheet to Professional IPAM

While a spreadsheet is excellent for learning and planning, in a production environment with thousands of IPs, you'd use a dedicated IPAM tool like NetBox. The manual process you just learned is critical for understanding what these tools do automatically.

These tools allow you to define your parent aggregate (10.50.0.0/16) and then create prefixes (your subnets) from it. The tool validates your work, preventing you from creating overlapping subnets and automatically tracking IP address utilization.

For someone comfortable on the CLI, a major benefit of these tools is their API and import/export functions. You could plan your entire network in a spreadsheet, save it as a CSV, and bulk-import it into the IPAM, which then becomes the source of truth for your automation scripts.

Exercise: Complete the AlphaCorp IPAM

Now it's your turn. Below is the complete, sorted list of requirements for AlphaCorp. Your task is to complete the IPAM document we started.

Parent Block: 10.50.0.0/16
Starting Point: Our last allocated subnet was for Engineering Users (10.50.3.0/24), which ends at 10.50.3.255. The next available address is 10.50.4.0.

Sorted Requirements List:

1. HQ Corporate Users: 438 hosts -> /23 (Allocated: 10.50.0.0/23)
2. Guest WiFi: 250 hosts -> /24 (Allocated: 10.50.2.0/24)
3. Engineering Users: 225 hosts -> /24 (Allocated: 10.50.3.0/24)
4. Voice: 113 hosts -> /25 (126 usable)
5. HQ WiFi: 100 hosts -> /25 (126 usable)
6. Engineering WiFi: 57 hosts -> /26 (62 usable)
7. Servers: 50 hosts -> /26 (62 usable)
8. Network Management: 19 hosts -> /27 (30 usable)
9. P2P Link: 2 hosts -> /30 (2 usable)

Your Task:

Create a Markdown table for requirements 4 through 9. For each one, determine the correct subnet allocation starting from 10.50.4.0 and fill in all the columns of the IPAM document. Assume the gateway is always the first usable IP address.

Here is the table structure for you to fill in:

Conclusion

In this lesson, you've performed the most critical task in IP address planning: mapping a list of requirements to a VLSM allocation and documenting it in a structured IPAM. This disciplined process transforms a chaotic list of needs into an orderly, efficient, and non-overlapping address plan.

Key Takeaways:

• The VLSM allocation process must be methodical: sort requirements from largest to smallest, then allocate subnets sequentially from your parent block.
• Each requirement (VLAN) maps to one row in the IPAM document.
• The IPAM document captures all essential data: the VLAN, the calculated subnet details (network, usable range, broadcast), the gateway IP, and contextual information like VRF and location.
• This filled-out IPAM is the master blueprint that will guide your CLI configurations.

In our next lesson, we will focus on verifying the IPAM plan for completeness and the absence of overlapping allocations. You'll learn how to audit your own work to catch common errors before a single line of configuration is written, ensuring your network deployment is smooth and error-free.