VLSM
VLSM Definition
VLSM, or Variable Length Subnet Masking, helps allocate IP addresses more efficiently. It breaks a block of IP addresses into smaller, tailored blocks called subnets based on the number of devices in different segments of the network. Network designers use a subnet mask to define the boundary of each subnet, showing which part of the address is for the network and which is for individual devices (hosts).
In practice, this means you can assign smaller subnets to smaller teams and larger subnets to larger teams, so there’s no IP address waste. This keeps the network cleaner, easier to manage, and less crowded.
Key Networking Terms You Should Know
To understand how VLSM works, you first need to know a couple of basic networking terms.
Subnet
A subnet is a smaller piece of a larger network. It’s created by dividing a big block of IP addresses into smaller blocks. You can think of it like breaking a large box of Legos into smaller, organized sets. Each new pile makes it easier to manage and organize a network.
The process of creating subnets is called subnetting.
Subnet Masks
A subnet mask is a 32-bit number that breaks an IP address into two parts:
- The network portion, which shows which subnet it belongs to.
- The host portion, which identifies a specific device within that subnet.
All devices within the same subnet share the same network portion, allowing them to communicate directly with each other without needing a router.
With VLSM, network designers use subnet masks of variable lengths to create subnets that fit each group’s size. A subnet mask is made up of 0s and 1s (binary format): the 1s mark the network part, and the 0s mark the host part. You’ll usually see it in a decimal format (255.255.255.0) or in CIDR notation (/24).
How Does VLSM Work?
VLSM lets you customize the size of each subnet based on how many devices you need to connect. To set it up, you need to:
- Figure out how many devices each subnet needs.
- Apply the right subnet mask to each subnet.
- Allocate the IP addresses.
- Set up a routing protocol (like OSPF, EIGRP, RIP v2, or BGP) to let devices from the different subnets communicate with each other.
For example, let’s say your company has four departments: Marketing, HR, Sales, and Product Management. You can use a Class C network, which gives you a total of 256 IP addresses to work with.
You can split this network into four subnets to match each department’s needs:
| Subnet | Number of Hosts | Subnet Masks (CIDR) | IP Address Range |
| Subnet A (Marketing) | 100 | /25 | 192.168.1.1 to 192.168.1.126 |
| Subnet B (HR) | 50 | /26 | 192.168.1.129 to 192.168.1.190 |
| Subnet C (Sales) | 25 | /27 | 192.168.1.193 to 192.168.1.222 |
| Subnet D (Product Management) | 10 | /28 | 192.168.1.225 to 192.168.1.238 |
Adjusting the size of each subnet makes sure each department has just enough IP addresses. This keeps your network efficient and organized.
VLSM vs FLSM
| VLSM | FLSM | |
| Subnet size | Varies between subnets | The same across all subnets |
| Subnet mask | Different for each subnet | Same for all subnets |
| IP address use | Uses IP addresses more efficiently | May waste IP addresses |
| Flexibility | High—tailored to each need | Limited—“one size fits all” |
| Use case | Modern networks with mixed needs. | Simple or legacy networks |
VLSM and traditional subnetting (also called Fixed Length Subnet Masking or FLSM) help you organize and manage your network, but they work in slightly different ways.
Traditional subnetting is more rigid. It breaks a network into subnets that are all the same size. This keeps things simple, but it can waste IP addresses if your departments don’t all need the same number of devices. FLSM is ideal for small businesses with one or two departments with similar needs.
VLSM provides more flexibility. It breaks a network into subnets of different sizes based on how many IP addresses each part of your network needs. This helps you save address space and manage the network more efficiently. It’s best suited for large companies where each team requires a different number of devices.
Why Is VLSM Useful?
With VLSM, you can:
- Use IP addresses more efficiently: Give each team the exact number of IP addresses they need. This prevents waste and keeps the network organized and easy to manage.
- Keep your network organized: Create subnets sized for each group so the network stays structured. For example, in a hospital, Emergency, Radiology, and Administration can run on their own subnet to easily separate sensitive data from general traffic.
- Improve network performance: Limit unnecessary traffic by keeping subnets the right size. This helps everything run faster and more reliably. This is crucial in areas where speed matters most, like your IT systems or emergency department.
- Make troubleshooting easier: Isolate issues more easily thanks to a clear subnet layout. If the HR team has a problem, you can check their subnet directly instead of scanning the entire network.
How to Use VLSM Efficiently
To get the most out of VLSM, you have to plan ahead and stay organized. Here are some practical tips to use VLSM efficiently:
- Plan ahead: Figure out how many devices you’ll connect and the number of subnets you’ll need to support them.
- Track assignments: Use tracking tools and spreadsheets to track which subnets are in use and which IP addresses are still available. This can help prevent overlap and errors.
- Use clear setup templates: Create standard templates for configuring routers, switches, and other devices to keep everything consistent.
- Keep detailed records: Write notes about IP address ranges, subnet masks, device settings, and anything else that might be relevant to your network. Good records can make troubleshooting and updates much easier.
- Train network administrators: Upskill your team on essential VLSM concepts and offer regular training to keep everyone up to date with best practices and technologies.
- Test before you go live: Run tests on your subnet design, routing configuration, and device communication to catch any issues early and avoid downtime.
What Routing Protocols Does VLSM Work With?
VLSM works well with most modern routing protocols, such as OSPF, IS-IS, EIGRP, RIPv2, RIPv3 and BGP. However, it doesn’t pair well with older routing protocols because they’re classful, which means they can’t interpret subnets of different lengths well.
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FAQ
VLSM stands for Variable Length Subnet Masking. It’s a way to split a network into smaller blocks of different sizes, called subnets. With VLSM, you can use IP addresses more efficiently and allocate bigger subnets to teams with more devices and smaller subnets to teams with fewer devices. This conserves IP addresses and makes the network organized and flexible.
Traditional subnetting (FLSM) breaks a network into equal-sized subnets, which can waste IP addresses if some groups need less space than others. VLSM splits the network into subnets of different sizes, which can save unused IP addresses and allow you to match subnet sizes to your network’s needs.
Yes, if you have multiple teams or departments that need different numbers of IP addresses. Using VLSM will help you avoid wasting IP addresses and make managing your network easier as it grows.
You need essential planning tools, like spreadsheets to track subnets, online calculators to work out the right subnet sizes, and diagramming tools to visualize how everything connects. These help you plan, assign, and manage your IP addresses clearly to prevent overlap.
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