Every device on the internet has an address, just like your house. The twist? It’s written back-to-front compared to what you’d expect.
Think about how you’d write your own mailing address. You start specific and zoom out: your name, your house number and street, then your city, then your state, then the zip code, narrowest detail first, broadest area last.
An IP address is a mailing address for a device on the internet, but it’s written in the opposite order. It starts broad and gets more specific as you go.
An IP address is a device’s location on the internet, written broadest-part-first: general area, then narrower, then narrower still, ending with the exact device.
An IP address looks like four numbers separated by dots, something like 192.168.1.42. Each of those numbers is called an octet, and each one narrows things down further than the last:
So when data goes looking for a device, it’s doing the same thing a delivery driver does with your address: first get to the right general area, then the right street, then the right house. The IP address is just that same funnel, written out as four numbers.
Each octet is 8 bits, so an IPv4 address is 32 bits total, which caps the whole address space at 232, about 4.3 billion. That ceiling is the entire reason IPv6 exists.
Where the “network” part stops and the “device” part starts isn’t fixed at the dots. It’s set by the subnet mask, usually written in CIDR notation as a suffix. In 192.168.1.42/24, the /24 means the first 24 bits (192.168.1) identify the network and the remaining 8 bits identify the host, giving you 254 usable addresses. Change it to /25 and you’ve split that into two smaller networks of 126 hosts each. Two addresses in every subnet are always spoken for: the all-zeros network address and the all-ones broadcast address.
Two devices can only talk directly if they’re on the same subnet. Anything else gets handed to the default gateway (your router), which forwards it onward. That single decision, “same subnet or not,” is what your device runs on every outbound packet.
Reading an IP address is like reading your own mailing address backwards, zip code first, house number last.
Here’s a wrinkle worth knowing: not every IP address can be reached directly from the internet. Some are private addresses, numbers set aside (in a rulebook called RFC 1918, if you ever want to impress someone at a party) for use only inside a home or office network. They never route out to the wider internet at all.
Think of it like an apartment building. The building itself has one address that the outside world (the postal service, delivery drivers, the internet), actually knows how to reach. But once mail arrives at the building, it’s useless without an apartment number. Each resident inside has their own private number that only means something inside that building. Nobody outside can mail something directly to “Apartment 4B”. They can only reach the building, which then routes it the rest of the way.
Your home network works the same way. Your router has the one address the internet can actually reach, the “building.” Your laptop, phone, and smart TV are the apartments: each has its own private, internal-only address, invisible from outside, that only matters for getting things to the right resident once they’ve already arrived at your router’s front door.
RFC 1918 reserves three ranges that public routers will never forward: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. That’s why nearly every home network you’ve ever joined started with 192.168. or 10.. Everyone is reusing the same private numbers behind their own front door, with no conflict, because those numbers never leave the building.
The translation layer is NAT (Network Address Translation). Your router rewrites the source address on outbound packets to its own public IP, records the mapping in a translation table, and reverses the swap on the way back. Because many internal devices share one public address, the router disambiguates replies by port number, properly called PAT, though almost everyone just says NAT.
This is also why inbound connections don’t “just work”: nothing outside can initiate a conversation with a device that has no publicly routable address. Getting around that is exactly what port forwarding, UPnP, and VPNs are for.
You’ll sometimes hear people mention IPv6, with addresses that look far longer and messier. Same idea, just a much bigger pool of numbers. We ran low on the short ones, so we printed more.
Which raises a fair question: who’s handing out all those apartment numbers, and who’s making sure two residents don’t end up with the same one? That’s a job for something called DHCP, coming up next.
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