Unit 6 · The Internet & Computing Systems · Lesson 6.3

Addresses and Names

Every device on the internet has a numeric address. You have never typed one. Today: what the number is, why the world ran out of them, and the phone book that hides them from you.

Big Idea 4 — Computer Systems & NetworksCED 4.1 · callback to 1.3

Before this lesson

No reading — this page is the reading. Have Lesson 1.3 (overflow) in mind: an IP address is a fixed number of bits, and you already know what happens when a fixed number of bits runs out.

Where We Are

People→ Bits→ Programs→ Internet→ Security→ Big Data→ Impact

Internet. The internet layer’s job is addressing. Today it meets Unit 1: an address is just a binary number with a fixed width.

The Hook

You’ve never typed one of these. 203.0.113.42

That’s what an internet address looks like. Every website you visit has one. So why do you only ever type names like example.com?

The Idea

An IP address is a unique number assigned to a device on the internet, so packets know where to go. The original version, IPv4, uses 32 bits, written as four numbers from 0 to 255 (each one is 8 bits).

32 bits gives 2³² = 4,294,967,296 addresses — about 4.3 billion. In 1981 that sounded infinite. Then came phones, laptops, tablets, game consoles, doorbells, and fridges. There are about 8 billion people — roughly half an address per person. IPv4 ran out.

The fix is IPv6: 128 bits. That’s 2¹²⁸ addresses, a 39-digit number. We will not run out.

It’s Lesson 1.3 again, for the whole planet. A fixed number of bits can only count so high. The fix then was more bits; the fix now is more bits. Each extra bit doubles the number of addresses, so going from 32 to 128 bits isn’t 4 times as many — it’s 2⁹⁶ times as many.

And the names? The Domain Name System (DNS) is the internet’s phone book. You type a domain name; DNS looks up its IP address; your device connects to the number. DNS is hierarchical: no single server knows every name, so the lookup asks its way down, level by level.

Try It — Count the Addresses, Then Look One Up

Part 1. Pick an address width. The count is exact — computed by your browser.

address bits:

Part 2. Your browser needs the address of photos.example.com. Step through the lookup.

Addresses on this page come from ranges reserved for examples, so they don’t point to anyone’s real server.

Vocabulary

IP address Internet: A unique number assigned to each device on the internet so that data can be sent to it.
IPv4 Internet: The original address format: 32 bits, about 4.3 billion addresses. It has run out.
IPv6 Internet: The newer format: 128 bits — enough addresses that running out is not a realistic concern.
DNS Internet: Domain Name System. The hierarchical system that translates domain names into IP addresses.
domain name Internet: The human-readable name of a site, like example.com.

Check

Five questions. Two need Unit 1 arithmetic — powers of two.

0 of 5 answered

Impact Check

The phone book can be a choke point

Because every name lookup goes through DNS, whoever runs the DNS server your device uses can see which sites you look up — and can refuse to answer for some of them. Some schools, networks, and governments block sites exactly this way.

Connections

Came from 1.3 — overflow. A fixed number of bits runs out; IPv4 exhaustion is that lesson at planet scale.

Came from 1.2 — binary: each part of an IPv4 address is one byte, 0–255.

Next in 6.4 — the address goes on every packet. What else does a packet carry?

Returns in 6.11 — DNS is the first step of loading any web page.