Type bbc.co.uk into a browser and a page appears in under a second. But your computer has no idea what "bbc.co.uk" means — it only knows how to send numbered chunks of data, called packets, to numbered destinations. So between you pressing Enter and the page loading, three different kinds of "address" quietly do their jobs. Getting them straight is one of the most reliable ways to pick up marks in a GCSE networking exam, because it is also one of the most commonly muddled topics.

The three ideas on this page:

• a MAC address — a permanent hardware label baked into a device;
• an IP address — a device's location on a network, which can change;
• the DNS — the system that turns a human name into an IP address.

A useful picture to keep in your head: sending data across the internet is a lot like sending a parcel. The MAC address is like the serial number stamped on the delivery van (it never changes, and only matters on the local road); the IP address is like the postal address on the parcel (it says where to deliver, and it can change if you move house); and the DNS is like a phonebook or directory you look a name up in to find that postal address in the first place.

The MAC address — a device's permanent name

Every device that can join a network has a network interface card (NIC) — the bit of hardware that does the actual sending and receiving. When that card is manufactured, a unique MAC address (Media Access Control address) is burnt into it. It is fixed hardware: it identifies the device itself, not where the device happens to be.

A MAC address is 48 bits, usually written as six pairs of hexadecimal digits, like this:

The first half identifies the manufacturer of the card, and the second half is a serial number that manufacturer assigns — together they are meant to be unique to that one card in the whole world. Because it never changes, the MAC address is what a device uses to recognise other devices on the same local network (for example, the machines joined to one home router). Think of it as the road on which everyone already knows each other's faces.

The IP address — a device's location

An IP address (Internet Protocol address) identifies where a device is on a network or on the internet. It is the address packets are actually routed to. Unlike the MAC address, it is not fixed to the hardware — it is assigned when the device joins a network, and it can change: connect your phone to your home Wi-Fi and it gets one IP; take it to school and it gets a different one.

The version you will see most in a GCSE exam is IPv4: four numbers, each from 0 to 255, separated by dots:

Each of those four numbers is one 8-bit byte, so an IPv4 address is 4 \times 8 = 32 bits in total. That gives about 4.3 billion possible addresses — which sounds enormous, but the world ran short of them, which is why the newer, much larger IPv6 (128 bits) was created.

Why have both a MAC and an IP if they both "identify" a device? Because they answer different questions. The IP address is logical and organised by location, so routers can work out a sensible path across many networks to reach it — just as the postcode system lets the postal service route a parcel across the country. The MAC address has no sense of location at all, so it is useless for routing between networks; it only sorts out "which device on this local network gets the packet" at the final step.

The DNS — the internet's phonebook

Humans are good at remembering names like bbc.co.uk or google.com. Computers need IP addresses. The Domain Name System (DNS) is the giant, distributed directory that bridges the two: give it a domain name and it hands back the matching IP address. That is why people call it "the internet's phonebook" — you look up a name to get the number you actually dial.

Follow the steps below. A device wants to visit bbc.co.uk but only has the name, so first it asks a DNS server, gets the IP address back, and only then can it send packets to the web server.

In real life the lookup is often more than one hop — your computer may ask a nearby DNS server, which asks bigger DNS servers higher up, until the name is resolved — but the core idea for the exam is exactly the four steps above: name → DNS server → IP address → connect to the web server.

Putting it together: what happens when you load a page

Trace a single web request end to end, and watch each address do its own job:

1. You type bbc.co.uk. Your computer doesn't know its IP, so it sends a DNS request asking "what is the IP address for this name?"
2. The DNS server replies with the IP address, e.g. \texttt{212.58.244.22}.
3. Your computer builds packets addressed to that IP. Routers across the internet use the IP address to route the packets, hop by hop, towards the right network — the same way a postcode routes a parcel.
4. On the final local network, the MAC address is used to deliver the packet to the exact physical device — the last step down the road to the correct front door.
5. The web server replies the same way, and the page loads.

Notice how neatly the three ideas divide the work: the DNS found the number from the name, the IP got the packets to the right place across the world, and the MAC handed them to the right device at the very end.