Definition
Dynamic DNS (DDNS) is a technology that links a computer's ever-changing internet address (its IP address) to an easy-to-remember domain name, in real time. Even on a home router or a home server, where the connection number changes from time to time, you can keep using the same domain name without worrying about address changes.
Why does your home IP address keep changing?
When you use the internet at home, your internet provider doesn't give every household a permanent number. It lends you a temporary one, called a dynamic IP address. Internet addresses are in limited supply. So if you restart your router, or your provider shuffles things around, your home's internet license plate changes.
When you browse the web or watch YouTube, a changing address causes no trouble at all. Your computer is the one that reaches out to the website first, so the site only needs to send its reply to your new address.
The trouble starts the other way around. Say you're away from home and want to reach your own computer, your home cloud storage (NAS), or your pet camera. To find your device from outside, someone needs to know your home address. But if your IP address changes without warning, the old address is useless. Nobody can knock on the right door anymore.
How does DDNS find the new address?
The regular internet phone book (standard DNS) was built for sites whose addresses almost never change, like big web portals. If an address does change, an administrator has to update it by hand. Then it can take hours or even days for the new information to spread to servers around the world.
With DDNS, a small program (an agent) on your router or computer acts as a watchman that works around the clock. It checks your internet address every short while. When it notices a change, it rushes to the DDNS server and registers the new IP address.
Thanks to this, you don't have to look up a long string of numbers every time. You just use one easy name, like "myhome.ddns.net." It works like a phone book that updates a friend's address by itself every time they move.
To be more precise: extra setup for reaching inside your home
To be more precise, what DDNS solves is finding the front gate of your home, which is the router's public IP address. But behind the router, many devices share a single internet line: your family's phones, laptops, TVs, and more.
So to guide a visitor at the front gate to one specific computer or file server inside, you also need a room-number setting called port forwarding.
DDNS also sets the lifetime of the address information (called TTL) very short, usually just a few minutes, so that changes show up quickly. That way, computers don't remember an old address for long. They ask for the latest one often, which keeps dropped connections to a minimum.
Common misconceptions
If you sign up for DDNS, your computer's IP address becomes a fixed IP that never changes.
Your computer's real IP address still changes from time to time. The difference is that every time it changes, DDNS instantly updates the number linked to your domain name. So the address only feels fixed from the outside.
Where you meet it
It's used to check a pet camera at home from your phone app whenever you're out.
It's used to open photos and documents on your own computer from outside, once you've set up a personal file server (NAS).
It's used when you host a Minecraft game server on your own computer so your friends can join.
In one sentence
It automatically updates your home's changing dynamic IP address to a domain name in real time, so you can always connect through one fixed name.