// article 02

What is an IP Address?

If the internet is a postal system for data, an IP address is the mailing address. Every device that wants to send or receive data needs one.

The basic idea

An IP address (Internet Protocol address) is a unique identifier assigned to a device on a network. When one device sends a packet, it stamps the packet with its own IP (the sender) and the IP of where it's going (the recipient). Routers along the way read the destination IP and forward the packet toward it.

Without IP addresses, packets would have no idea where to go — like dropping a letter in a mailbox with no address written on it.

IPv4: the classic format

The most familiar form is IPv4, written as four numbers separated by dots:

192.168.1.42

Each of the four parts (an "octet") ranges from 0 to 255, because each is 8 bits — and 8 bits can represent 256 values. Four octets give about 4.3 billion possible addresses. That sounded enormous in the 1980s, but we've since run out, which leads to the next format.

IPv6: the roomy upgrade

IPv6 uses much longer addresses written in hexadecimal:

2001:0db8:85a3:0000:0000:8a2e:0370:7334

It provides roughly 340 undecillion addresses — enough that we won't run out for the foreseeable future. The two formats coexist today; most networks speak both.

Public vs. private addresses

Here's a detail that confuses a lot of people: the IP your laptop sees for itself is usually not the IP the rest of the internet sees.

  • Private IPs are used inside your home or office network. Certain ranges are reserved for this, such as 192.168.x.x and 10.x.x.x. They're not unique globally — millions of homes reuse 192.168.1.1.
  • Public IPs are globally unique and assigned by your ISP. This is the address the wider internet uses to reach your network.

Your router bridges the two using NAT (Network Address Translation): it lets many private devices share one public IP, keeping track of which internal device each conversation belongs to.

Private IP = your apartment number inside a building. Public IP = the building's street address. The mail carrier (the internet) only needs the street address; the front desk (your router) handles the rest.

Static vs. dynamic

Most home addresses are dynamic — assigned automatically by a server using DHCP and able to change over time. Servers often use static IPs that never change, so they're always reachable at the same place.

Why this matters for security

  • IP addresses reveal rough geolocation and which ISP you use, which is why VPNs exist — they swap the public IP that services see.
  • Firewalls make allow/deny decisions based on source and destination IPs and ports.
  • Attackers scan ranges of public IPs looking for exposed services, so knowing what's reachable from the public internet is a core defensive habit.

But an IP address is just a number — humans prefer names like example.com. Translating between the two is the job of DNS.

← How the Internet Works  ·  Next: What is DNS? →