TUTORIAL
How to Select Subnet Sizes for VLANs
Project Overview
EXPERIENCE LEVEL: Intermediate
TIME TO COMPLETE: 40-50 minutes
ESTIMATED COST: Free, assuming you have access to your network equipment.
Skills Needed
Understanding of basic networking concepts (IP addressing, subnetting, VLANs)
Familiarity with subnet masks and CIDR notation
Experience navigating switch or router configuration interfaces
Tools and Materials Needed
Managed switch or router that supports VLANs
Access to your network configuration console (CLI or GUI)
IP addressing plan or documentation for your network
Calculator (optional, for subnet math)
Before You Begin
Before you get started, here are a few things to know:
Confirm you understand how VLANs and subnets relate; each VLAN typically maps to its own subnet.
Know your approximate number of devices per VLAN (including future growth).
Determine whether certain VLANs will support Bring Your Own Device (BYOD) or guest access—these often require additional IP addresses.
Have a basic IP addressing scheme ready (e.g., private IP ranges like 192.168.x.x or 10.x.x.x).
You'll also want to be familiar with the definitions of subnets and VLANs. Here's a quick rundown:
What is a Subnet?
A subnet is, essentially, a network inside a network. A subnet or "subnetwork" is a logical subdivision of an existing Internet Protocol (IP) network. Subnetting enables us to direct network traffic to its destination through alternative routes, over shorter distances, and without using unnecessary routers.
What is a VLAN?
A virtual LAN (VLAN) is a logical division of a computer network, partitioned and isolated within its network at the data link layer. With a VLAN, it's possible to combine physical devices (wired and wireless) from multiple networks and treat them as one logical network, even though they're physically connected.
How to Select Subnet Sizes for VLANs
Choosing the right subnet size for each VLAN ensures you have enough IP addresses for your devices without over-allocating space. This guide walks you through common subnet sizes, when to use them, and how to assign them to typical VLANs in a medium-sized network.
Here are the five subnets you’ll see most often, along with their typical use cases:
/22 (255.255.252.0): 1,024 IPs / 1,022 usable: Large client subnet
/23 (255.255.254.0): 512 IPs / 510 usable: Medium client subnet
/24 (255.255.255.0): 256 IPs / 254 usable: Small client subnet
/29 (255.255.255.248): 8 IPs / 6 usable: Redundant WAN uplinks
/30 (255.255.255.252): 4 IPs / 2 usable: Point-to-point WAN uplinks
Step 1: List all VLANs You Need
You can’t size a subnet if you don’t know what it’s for. Listing VLANs upfront ensures you design for actual needs, not vague guesses, and helps you avoid unnecessary VLAN sprawl later.
Write down each VLAN you plan to create and its purpose. Example:
VLAN 10: Servers & Printers
VLAN 20: VoIP
VLAN 30: Guest/BYOD
VLAN 40: Internal Managed Devices
VLAN 50: Flex/Lab
Step 2: Count Devices and Add Growth Buffer
Subnetting for only today’s devices is a recipe for headaches later. Adding a buffer prevents you from running out of IPs, which can force disruptive readdressing. Count the current devices (servers, printers, phones, laptops, IoT devices, etc.), then multiply by 1.5 to account for future growth. For example:
VLAN 20 (VoIP): 30 phones × 1.5 = 45 IPs needed.
Multiplying by 1.5 is a good rule of thumb, but some environments (like those with a lot of BYOD) might require 2X growth allowance.
Step 3: Match Device Counts to a Subnet Size
Different subnets offer very different amounts of usable IP addresses. Matching your counts to the right CIDR block keeps your network efficient and avoids wasting address space.
Use this quick reference:
/30 (255.255.255.252) → 2 usable IPs — Point-to-point links.
/29 (255.255.255.248) → 6 usable IPs — Redundant WAN uplinks.
/24 (255.255.255.0) → 254 usable IPs — Small subnets.
/23 (255.255.254.0) → 510 usable IPs — Medium subnets.
/22 (255.255.252.0) → 1,022 usable IPs — Large subnets.
Once you know your device counts and the matching CIDR block, it’s time to map them directly to your VLANs.
Step 4: Assign VLAN IDs and Subnets
Tying the right subnet to the right VLAN ensures devices get the addressing they need without interfering with other network segments. This step makes your design real.
Example plan for a 30-employee office:
VLAN ID | Purpose | Subnet | Usable IPs | Why This Size Works |
10 | Servers & Printers | /24 | 254 | Plenty for static IPs & network gear |
20 | VoIP | /24 | 254 | One phone per user + room for expansion |
30 | Guest/BYOD | /22 | 1,022 | Covers large spikes in personal devices |
40 | Internal Managed Devices | /22 | 1,022 | Multiple devices per user + IoT |
50 | Flex/Lab | /24 | 254 | Staging/testing without wasting space |
Step 5: Verify Your Addressing Plan
It’s tempting to jump straight into configuration once you’ve picked your VLAN subnets, but a quick verification step can save you from network disasters later. Overlapping subnets, incorrect ranges, or mismatched VLAN assignments can lead to routing loops, IP conflicts, or outages that are a nightmare to troubleshoot.
Make sure all VLAN subnets fit neatly within your chosen private IP range (e.g., 10.x.x.x or 192.168.x.x).
Check that no two VLANs share the same subnet. Overlapping subnets cause routing conflicts because devices and routers can’t determine the correct path for traffic, which can lead to packet loss and outages.
Confirm that your chosen subnet sizes match your documented device counts and growth plan.
Update your network diagram and IP address documentation so future changes don’t break your plan.
Step 6: Configure VLANs on Your Equipment
All the planning in the world won’t help if your configuration is inconsistent. Misconfigured VLANs can cause devices to lose connectivity, create security holes, or block traffic entirely. This step ensures your VLAN IDs, subnets, and gateways are correctly set up on every relevant switch, router, and firewall.
Create each VLAN in your switch/router configuration.
Assign the correct IP subnet and default gateway to each VLAN interface.
Apply the right trunk/access port settings so devices are placed in the intended VLAN.
Test connectivity within each VLAN to ensure that devices can reach one another.
If inter-VLAN routing is required, test communication between VLANs according to your network security policies.
Final Thoughts
Choosing the right subnet size for each VLAN ensures your network runs smoothly today and scales easily tomorrow. Smaller subnets (/24) work well for predictable device counts, while larger subnets (/22, /23) are suitable for high-demand networks, such as guest Wi-Fi or managed devices.
Want to learn more? Check out our online course on IPv4 Subnetting. Or, find more step-by-step guides on our tutorials page.