Theory
How data finds your device
For your request to reach a specific website, and for the reply to come back to your specific laptop, each device needs a unique address, just as a letter needs a postal address. On a network, that identifier is the IP address.
IP addresses are what the network layer uses to route data to the right place. This lesson covers what an IP address is, the two versions in use (IPv4 and IPv6), and the difference between public and private addresses. It is a short but foundational idea.
Theory
IPv4: four numbers, 0 to 255
The long-established version is IPv4. An IPv4 address is a 32-bit number, written for humans as four decimal numbers separated by dots (dotted-decimal), like 192.168.1.1.
Each of the four parts is called an octet (8 bits), and because 8 bits can represent 256 values, each octet ranges from 0 to 255. So 192.168.1.1 is valid, but something like 300.1.1.1 is not, 300 is out of range. With 32 bits, IPv4 provides about 4.3 billion possible addresses, which once seemed endless but no longer is.
At a glance
| Idea | Detail |
|---|---|
| IPv4 | 32-bit; four octets 0-255 in dotted decimal (e.g. 192.168.1.1); about 4.3 billion addresses |
| IPv6 | 128-bit; eight hexadecimal groups; a vastly larger address space |
| Public IP | Globally unique; reachable across the internet |
| Private IP | Used within a local network (e.g. 192.168.x.x); not routed on the public internet |
Formula
Why IPv6 exists
IPv4's roughly 4.3 billion addresses sounded limitless in the early days, but with billions of phones, computers, and internet-connected devices, the world has effectively run out of fresh IPv4 addresses.
IPv6 was created to solve this. By using 128 bits instead of 32, it offers an astronomically larger pool of addresses, enough for every device imaginable, far into the future. The two coexist today, with the internet gradually adopting IPv6, but the core idea is the same: a unique address so data can find each device.
Quiz
Which of these is a valid IPv4 address?
- 256.10.10.1, because IPv4 allows any numbers
- 192.168.1.1, four octets each in the range 0 to 255
- 192.168.1, because three numbers are enough
- Hello.World.1.1, using words
Show the answer
192.168.1.1, four octets each in the range 0 to 255
An IPv4 address is four decimal octets, each from 0 to 255, separated by dots, so 192.168.1.1 is valid. Option A is invalid: 256 is out of range, an octet is 8 bits, allowing only 0 to 255, so the maximum is 255 (256 would need a 9th bit). Option C is incomplete: IPv4 needs FOUR octets, not three; 192.168.1 is missing a part. Option D is nonsense: IPv4 uses numbers, not words. Remember the format: four numbers, each 0 to 255, dot-separated, which is why checking octet ranges quickly spots invalid addresses like 256.x.x.x or 300.x.x.x.
Think first
Why do private IP addresses like 192.168.x.x exist?
If every device needs a unique address, why are addresses like 192.168.1.1 reused on millions of home networks? Then tap.
Show the answer
Because private addresses let many local networks REUSE the same address ranges internally, which conserves the scarce supply of globally unique public addresses. There simply are not enough IPv4 addresses to give every single device its own globally unique one, so the design splits addresses into PUBLIC (globally unique, routable across the internet) and PRIVATE (reserved ranges like 192.168.x.x meant only for use inside a local network). Your home or campus network hands out private addresses to all its devices, and those addresses are NOT routed on the public internet, so it does not matter that a million other homes use 192.168.1.1 too; each is meaningful only within its own local network. When a device needs to reach the internet, a router performs Network Address Translation (NAT), letting many devices SHARE one public address on the outside while keeping their private addresses on the inside. This is a major way IPv4 has stretched its limited supply: instead of every device consuming a public address, whole networks hide behind one. It also adds a measure of separation, since private addresses are not directly reachable from outside. So private addresses solve a scarcity problem elegantly: reuse the same internal numbers everywhere, and translate to a shared public address only when leaving the local network. Reuse inside, translate on the way out.
Summary
Key takeaways
- An IP address is a unique numeric identifier for a device on a network, used by the network layer to route data to it.
- IPv4 is 32-bit, written as four decimal octets (0 to 255) separated by dots, e.g. 192.168.1.1.
- Each octet ranges 0 to 255 (8 bits), so 256.x.x.x is invalid; IPv4 has about 4.3 billion addresses.
- IPv6 is 128-bit with a vastly larger address space, created because IPv4 addresses are running out.
- Public IP addresses are globally unique and reachable on the internet; private addresses (like 192.168.x.x) are used within local networks.
- Private addresses can be reused across many local networks and are translated (NAT) to a shared public address to reach the internet.
- Memory hook: IPv4 is four numbers 0 to 255; IPv6 is the much bigger successor.