Network+ (N10-009) · ELI Explains IP Addresses (book)
Understanding IP Addresses Before Using This Guide
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Every device on a network — your phone, laptop, smart TV, or a web server halfway around the world — needs a way to be found and identified. That identifier is called an IP address.
The college version
What Is an IP Address?
An IP (Internet Protocol) address is a unique number assigned to each device connected to a network that uses the Internet Protocol. Think of it as a device's network mailing address. Just as a postal service needs your street address to deliver a letter, networks need an IP address to deliver data to the right device.
There are two versions of IP addresses in use today: IPv4 and IPv6.
IPv4 at a Glance
IPv4 is the older and still widely used version. An IPv4 address is a 32-bit number, usually written as four decimal numbers separated by dots — like 192.168.1.1. Each number ranges from 0 to 255. With 32 bits, IPv4 can represent about 4.3 billion unique addresses. That seemed enormous when the internet was designed, but it's no longer enough for the billions of devices connected today.
IPv6 at a Glance
IPv6 is the newer version, designed to solve the address shortage. An IPv6 address is a 128-bit number, written as eight groups of four hexadecimal digits separated by colons — like 2001:0db8:85a3:0000:0000:8a2e:0370:7334. The address space is staggeringly large: about 340 undecillion addresses. IPv6 also introduces improvements like simpler packet headers and built-in support for automatic address configuration.
Why Two Versions?
IPv4 addresses ran out not because they stopped working, but because the world has more internet-connected devices than IPv4 has addresses. IPv6 was created to provide enough addresses far into the future and to improve how networks handle traffic. The two versions coexist, and most modern networks run both simultaneously — this is called dual-stack operation.
Public and Private Addresses
Not every IP address is visible to the entire internet. Public addresses are globally unique and routable — they're what web servers, cloud services, and your home router's internet-facing side use. Private addresses are reserved for use inside local networks: your home Wi-Fi, office LAN, or school network. They cannot travel directly across the public internet. Instead, a technique called Network Address Translation (NAT) lets many private-address devices share one public address.
Addresses, Networks, and Hosts
An IP address has two parts: a network portion that identifies which network the device belongs to, and a host portion that identifies the specific device on that network. A subnet mask or CIDR prefix length (like /24) tells a device where the boundary is. This is how your phone knows that your laptop is on the same local network but a website is somewhere else entirely.
What an Address Does Not Tell You
An IP address alone does not prove a device's identity, location, or security posture. Addresses can be shared (via NAT), spoofed, changed dynamically, or assigned temporarily. A private address does not guarantee protection, and a public address does not mean exposure. Addresses are identifiers — not security guarantees.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine every house in the world needs a unique mailing address so the mail carrier knows where to deliver letters. That's what an IP address does for computers. IPv4 addresses are like having only 4.3 billion house numbers — we ran out! So engineers created IPv6, which is like adding a whole new numbering system with so many addresses we won't run out for a very, very long time. Private addresses are like room numbers inside your house that only your family uses. Public addresses are like your street address that anyone in the world can use to find you.
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