DHCP Pool Utilization Calculator

It’s always at some crucial time. It happens when you’re deploying a container, when you clone a virtual machine, or when you connect a smart plug that doesn’t get its IP address assigned by your home lab. Panic sets in. You check the router and there it is, a silent exhaustion of resources! You’ve run out of address space! And this points out how static address space simply isn’t appropriate for our dynamic world.

Most people think “oh well, I got a /24 subnet; that’ll last forever!” No, it won’t last forever and you probably is depleting what little you have much quicker then you realize. Usually, the problem isn’t that there aren’t any addresses at all. It’s that there are lots of addresses… but not always very many free ones. Every connected device take one out of pool, whether it uses it for an hour or just for a second. When devices lose connection (or go to sleep) without first disconnecting cleanly, those addresses sits in the server’s hands until their time runs out.

How to Plan Your IP Addresses

And then there are your persistent devices, your NAS units, your printers, etc… That has reserved spots for themselves permanently. Those aren’t going anywhere, which means less pool for everyone else to use. Keep your guest/device/other temporary needs separated from your hard-wired infrastructure.

After plugging in your active leases and scope size, the calculator do the rest for you. It takes out all of the guesswork when it comes to conversions and coefficients. It forces you to visualize what is actualy usable versus how much total capacity you have. A lot of folks will glance at the overall amount of hosts and forget about the exceptions. Take away the reserved static IP‘s, the broadcast address, and the network address. That leave a huge decrease in the available number. You’ll also notice that the percentage used of the dynamic pool isn’t as high than the theoretical maximum.

Many administrators don’t touch lease duration until they run into trouble. A short lease, like four or eight hours for guest Wi-Fi, sends addresses back to the pool rapidly. This is great if devices tends to connect and disconnect frequently. A long lease, like three to seven days for IoT sensors, minimizes server load and protocol chatter. But a long lease will mask connections that go inactive. You’re stuck with a tradeoff: How much do you want to minimize server load vs how much do you need to clean up network address usage? Optimize for one or the other depending on what kind of traffic you have.

Well planned networks get undermined by growth. Sure, everything fits today. But what about next month? Maybe you buy 15 new devices? Maybe you move into a larger virtualized environment? What about users? What about visitors? Phone usage might surge out of nowhere. The tool predicts out growth for thirty days and then throws in a little bit of safety buffer to see if it can fit within the scope you’re at right now. Better to find out you’re low before you lose your service.

To see what different systems will support, we have some reference tables of different home router and server systems. Enterprise servers will be able to manage their lease database more effective than a home router, for example. Understanding the limitations of your system is an important step in determining if you want to adjust the configuration or expand the network.

Generally, if the expected use exceeds 85% you’re going to want to split up the network. Most of the time it’s better to divide a crowded /24 into two subnets (/25). Do this instead of simply increasing the lease duration.

Planning for IP addresses means predicting how they’ll be used, it’s more than arithmetic. It’s a game of connecting dots: handling a device’s life cycle end-to-end. Matching leases with churn is key; making sure reservations remains useful. This is the result. New devices will have an address instanly, no problems.