Dynamic IP

Here's a math problem: there are roughly 4.3 billion IPv4 addresses in existence. There are over 15 billion internet-connected devices on the planet. Those numbers don't add up, and they haven't for decades.

So how does the internet still work? Because most devices don't have a permanent address. They borrow one when they need it and give it back when they're done. Your phone probably had three different IP addresses today and you didn't notice.

This is what dynamic IP addressing does. Instead of permanently reserving an address for every single device, a system called DHCP hands out addresses from a shared pool, recycles them when devices disconnect, and reuses them for new devices. It's the same principle as a hotel room, you don't own room 412 forever, you rent it for a few nights, and when you leave, someone else moves in.

Without dynamic IPs, the internet would have run out of addresses sometime around 2005 and everything would have ground to a halt. Dynamic addressing is the duct tape that held IPv4 together long enough for IPv6 to (eventually, slowly, painfully) roll out.

When your device connects to a network, it doesn't just magically get an IP address. There's a four-step negotiation called the DHCP handshake, and it happens in under a second:

1. DISCOVER. Your device shouts into the network: "Hey, is there a DHCP server here? I need an address."
2. OFFER. A DHCP server responds: "I have 192.168.1.47 available. Want it?"
3. REQUEST. Your device replies: "Yes, I'll take 192.168.1.47."
4. ACKNOWLEDGE. The server confirms: "It's yours. Lease expires in 24 hours."

That "lease" part is key. Your device doesn't own the IP, it rents it. When the lease expires, your device has to renew it or get a new one. Most of the time renewal works fine. But if you disconnect from the network and come back later, you'll likely get a completely different address.