One address, lots of programs

A block of flats has one street address, but dozens of people live there. A parcel needs the street address and a flat number, or it sits in the hall.

A computer is the same. Its IP address (lesson 3.1) gets a message to the right computer. But on that computer a web browser, an email app and a video call are all running at once, and the message is for one of them. The flat number is the port number: a number from 0 to 65535 that says which program.

You met ports in lesson 1.2: they’re layer 4, transport. And in lesson 4.1, DHCP’s messages went to port 67 on the server and back to port 68 on the laptop.

in one sentence The IP address finds the computer; the port number finds the program on it.

Listening, and source and destination ports

A program that offers a service, like a web server, listens on a port: it tells the computer anything that arrives for port 443 is mine. Everyone knows the number in advance, so nobody has to ask.

Every message carries two ports, like it carries two IP addresses:

That temporary source port is called an ephemeral port (ephemeral means short-lived). When the server answers, it swaps them round: the answer comes from 443 and goes to 51000, so the laptop knows which program, and which conversation, it’s for.

Which numbers count as spare depends on the system. Windows and macOS pick from 49152 to 65535; Linux, by default, from 32768 to 60999.

in one sentence The destination port is the service’s well-known number; the source port is a temporary one the asking computer picks.

Sockets

An IP address and a port together are called a socket, written with a colon: 192.168.1.10:51000. A conversation between two programs is two sockets, one at each end:

192.168.1.10:51000  ⇄  192.168.2.10:443
   the laptop            the web server

Open a second tab to the same site and you get a second conversation, 192.168.1.10:51001 ⇄ 192.168.2.10:443. The server’s side is the same; the laptop’s port is different. That’s how one computer keeps dozens of conversations apart.

in one sentence A socket is an IP address plus a port; a conversation is a pair of sockets.

Two ways to send: TCP and UDP

Layer 4 has two main protocols, and every port number belongs to one of them (or to both, separately: TCP port 53 and UDP port 53 are two different doors).

Voice and video use UDP for a reason: a bit of sound that arrives late is no use, so there’s no point sending it again. Better to skip it and carry on.

in one sentence TCP sets up a connection and makes sure everything arrives; UDP just sends, and is quicker for it.

The three-way handshake

Before TCP sends any data, the two sides agree to talk. Three messages, named after the flags they carry (a flag is a switch in the TCP header, on or off):

1. SYN (synchronise): the laptop says I’d like to talk, and I’ll number my pieces starting here.
2. SYN-ACK: the server says OK (ACK, acknowledge) and here’s where my numbering starts (SYN).
3. ACK: the laptop says got it.

Now the connection is established, and data flows both ways. Every piece is numbered, and the other side acknowledges what it got, so anything missing is noticed and sent again. When they’re done, each side sends a FIN (finish) to close it.

in one sentence SYN, SYN-ACK, ACK: TCP’s hello, before any data.

Three ranges

The numbers 0 to 65535 are split into three ranges. IANA, the Internet Assigned Numbers Authority, the organisation that keeps the official list, writes down who each number is for.

The new student

It’s a new student’s first day. They don’t know anyone or where to sit. So they stand at the front and ask the whole room, “Where can I sit?” The teacher walks over and says, just to them, “Desk 10 is free.” The student says, out loud to everyone, “I’ll take desk 10!” And the teacher says, just to them, “It’s yours, for this term.”

A brand-new laptop plugged into a network is that new student. It has a MAC address (its name, lesson 1.1) but no IP address (where it lives, lesson 3.1), so it can’t talk to anything beyond the end of its cable.

in one sentence A new laptop is like a new student: it has a name, but no seat yet.

DHCP: the teacher who hands out seats

Nobody wants to type addresses by hand into every phone and laptop. So one computer on the network has the job of handing them out: the DHCP server. DHCP stands for Dynamic Host Configuration Protocol. Taking it apart:

• Dynamic: automatic, done as needed.
• Host: a laptop, phone or anything else that sends its own messages (lesson 1.1).
• Configuration: its settings.
• Protocol: the agreed rules (lesson 1.2).

So: the rules for giving hosts their settings automatically. At home, the DHCP server is inside your home router. At school or in an office, it’s often a server in a data center (lesson 2.3).

in one sentence A DHCP server hands out addresses and settings to new hosts automatically.

Shouting and whispering

There are two ways to send a message, and DHCP uses both:

• Broadcast: to everyone on the network, like the student asking the whole room. In a frame, the broadcast MAC address is FF:FF:FF:FF:FF:FF, and the broadcast IP address is 255.255.255.255. Both mean all of you.
• Unicast: to one host only, like the teacher talking to just the student. It uses that host’s own address. (Uni means one, like a unicycle has one wheel.)

in one sentence A broadcast goes to everyone on the network; a unicast goes to just one host.

DORA: four messages

The laptop and the DHCP server swap four messages, and their first letters spell DORA:

Ack is short for acknowledge: to say yes, got it.

in one sentence Discover (shout), Offer (to you), Request (shout), Ack (to you): DORA.

What the laptop gets

It doesn’t just get an address. It gets four settings:

1. its IP address, like 192.168.1.10;
2. the subnet mask, which says which addresses are on its own network (lesson 3.2);
3. the default gateway: the router, the way out to other networks (lesson 2.1);
4. a DNS server. DNS stands for Domain Name System: the internet’s phone book. People use names like networkdoodles.com, but computers need IP addresses, so the DNS server looks names up and gives back the address.

It gets them for a while, not forever, like borrowing a library book. That loan is called the lease. Halfway through, the laptop asks to keep its address for longer, and usually the answer is yes.

in one sentence The laptop borrows an address, a mask, a gateway and a DNS server, for the length of a lease.

Sharing a pizza fairly

Five friends share a big pizza. One is starving, one is quite hungry, and three just want a nibble. If you cut it into five equal slices, the starving one goes hungry and the nibblers leave crusts. If you cut big slices for the hungry ones and small slivers for the rest, everyone gets what they need and there might even be some left over.

in one sentence Give each person the size of slice they need, not the same slice for everyone.

Subnets

A school or company is usually given one block of addresses, like 192.168.10.0/24 (256 addresses: from lesson 3.2). Cutting that block into smaller networks is called subnetting, and each small network is a subnet (sub means under or part of, like a submarine goes under the sea).

Why cut it up?

• Groups: students, teachers and cameras each get their own subnet, so each can have its own rules.
• Fewer announcements: each subnet is its own broadcast domain (lesson 2.2), so broadcasts stay smaller.
• No waste: each group gets the size it needs.

in one sentence Subnetting cuts one block of addresses into smaller networks called subnets.

VLSM: a different size for each

The simple way is to cut equal pieces, but groups aren’t equal: 100 students and 2 addresses for a cable between two routers. Equal pieces big enough for the biggest group would waste nearly all of the small ones.

VLSM stands for Variable Length Subnet Masks. Variable means can change, and the length is the prefix from lesson 3.2. So: each subnet gets its own prefix, just big enough for its group. That’s the pizza cut to fit.

in one sentence VLSM gives each subnet its own prefix, so each group gets the size it needs.

Sizes come in doubles

Subnets can’t be any size: they come in powers of two, numbers you get by doubling, starting from 1: 2, 4, 8, 16, 32, 64, 128, 256. That’s because each host bit doubles the addresses (lesson 3.2). And remember, every subnet loses two addresses: the network address and the broadcast.

in one sentence A subnet’s size is a power of two, and two of its addresses are always taken.

The recipe

1. List the groups, biggest first.
2. For each group, add 2 (for the network address and the broadcast).
3. Find the smallest power of two that’s at least that number. That’s the block size. Which power it is, is the number of host bits: 64 = 2⁶, so 6 host bits.
4. The prefix is 32 minus the host bits: 32 − 6 = /26.
5. Place it at the next free address. A block has to start at a multiple of its own size (a block of 64 starts at 0, 64, 128 or 192). Going biggest first means it always does, with no gaps.
6. Write down the network (the first address), the hosts (everything between) and the broadcast (the network plus the block size, minus 1).

in one sentence Biggest first, add 2, round up to a power of two, place it at the next free address.

How long is the street name?

Picture a long road with two different signs. On one road, the street is called Oak and the houses are numbered 1 to 250. On another, the street is called Oak Avenue North and there are only 6 houses. The longer the street’s name, the fewer houses fit on it.

In lesson 3.1 you saw that an IP address is a network part (the street) and a host part (the house number). But 192.168.1.10 on its own doesn’t say where one ends and the other starts. That’s what the number after a slash is for:

192.168.1.10/24

The /24 is the prefix length. It means the first 24 bits are the network part. An address has 32 bits, so the other 32 − 24 = 8 bits are the host part.

in one sentence The number after the slash says how many bits, from the left, are the network part.

CIDR: just the slash

Writing networks as an address, a slash and a number is called CIDR (say cider). It stands for Classless Inter-Domain Routing. That’s a long name for a simple idea: say exactly how long the network part is, with a slash. (Long ago, networks only came in three fixed sizes called classes, which wasted lots of addresses. Classless means any size you like.)

in one sentence CIDR is the way of writing a network with a slash, like 192.168.1.0/24.

The subnet mask

There’s a second way of writing the same thing, which you’ll see on computers and phones: the subnet mask. Think of a stencil laid over the address: the holes show the network part, and the solid bits cover the host part.

To make it, write a 1 for every network bit and a 0 for every host bit, then turn each group of 8 into a number, like the light switches in lesson 3.1:

/24  =  11111111.11111111.11111111.00000000  =  255.255.255.0

So 192.168.1.10/24 and 192.168.1.10 with mask 255.255.255.0 mean exactly the same thing.

in one sentence The subnet mask, like 255.255.255.0, says the same thing as the slash, written as four numbers.

Two addresses nobody can have

Every network keeps two of its addresses for special jobs:

• the network address: the very first one, with every host bit 0. It’s the name of the whole network, like the name of the street, for example 192.168.1.0.
• the broadcast address: the very last one, with every host bit 1. Send a packet to it and everyone on the network gets it (the broadcast from lesson 2.2), for example 192.168.1.255.

So the hosts get everything in between.

in one sentence The first address names the network and the last one means everyone, so hosts get the ones in between.

How many hosts fit?

Every extra host bit doubles the number of addresses: 1 bit gives 2, 2 bits give 4, 3 bits give 8, and so on. Mathematicians write that as 2 to the power of the number of bits: 2⁸ = 256. Take away the two special addresses, and you’ve got the number of hosts:

Notice: every time the prefix goes up by one, the network is cut in half. Longer street name, fewer houses.

in one sentence Hosts = 2 to the power of the host bits, minus 2; each step bigger in the prefix halves the network.

Why a host cares

Before sending anything, a host asks itself: is this address on my network? It compares the network part of its own address with the network part of the address it’s sending to. Same street? Send it straight there. Different street? Send it to the default gateway (the router, the way out, from lesson 2.1).

in one sentence Same network part: send it straight there; different: send it to the gateway.

Your name and where you live

You have a name, and it goes wherever you go. You also have a home address, and that changes when you move house. If a friend in another country wants to post you a letter, your name alone isn’t enough: the post office needs the address, with the country, town and street, to know where to take it.

Every host already has a MAC address (from lesson 1.1): that’s its name. It never changes, and it’s only used to get a frame to the next stop on the same network (layer 2).

So hosts get a second address, an IP address: that’s their home address. It depends on which network they’re plugged into, and it’s used to get a packet across the world, through lots of networks and routers (layer 3). IP stands for Internet Protocol: the agreed rules (a protocol) for addressing and delivering packets across the internet.

in one sentence A MAC address is a host’s name; an IP address is where it lives right now.

What one looks like

The IP addresses in this course are IPv4 addresses (Internet Protocol version 4). They look like this:

192.168.1.10

Four numbers with dots between them. Each number can be anything from 0 to 255, so 192.168.1.300 isn’t a real address. Each of the four numbers is called an octet (oct means eight, like an octopus’s eight arms), because inside the computer each one is made of 8 bits. Four octets make 32 bits in all.

in one sentence An IPv4 address is four numbers from 0 to 255 with dots between, like 192.168.1.10.

Numbers as light switches

Remember that a bit is a 1 or a 0, like a light switch that’s on or off. Imagine a row of 8 light switches, and each switch is worth a different amount: 128, 64, 32, 16, 8, 4, 2 and 1. To make a number, switch on the ones that add up to it. That’s called binary: counting with only 1s and 0s.

That’s why 255 is the biggest: it’s all eight switches on.

in one sentence Each octet is 8 bits, and binary turns them into a number by adding up the switches that are on.

The street and the house number

A home address has a street (shared by everyone who lives on it) and a house number (different for each house). An IP address is split the same way:

• the network part is the street: every host on the same network has the same one.
• the host part is the house number: different for every host on that network.

For 192.168.1.10 on a home network, 192.168.1 is the network (every device at home starts with it) and 10 is this host. Exactly where the line between the two goes is the next lesson.

in one sentence The front of an IP address says which network, the end says which host on it.

Private and public addresses

Think of a hotel. Inside, the rooms have numbers, like room 214, and every hotel in the world has a room 214. But to post a letter to the hotel, you use its street address, which is unique. The front desk takes the letter and brings it up to the right room.

IP addresses work the same way. Some are kept for use inside homes, schools and companies, and are never used out on the internet. They’re called private addresses (the room numbers):

Every other address is public: a real, unique address out on the internet (the street address). Millions of homes all use 192.168.1.something inside, and that’s fine, because the home router acts like the front desk: it swaps the private address for the home’s one public address on the way out, and back again on the way in. That swap is called NAT, network address translation (translation means changing one thing into another).

in one sentence Private addresses are used inside a home or school; public addresses are unique on the internet; the router swaps between them.

And IPv6?

There are only about 4 billion IPv4 addresses, and that isn’t enough for every phone, laptop and smart light bulb on Earth. So there’s a newer version, IPv6 (Internet Protocol version 6), with much longer addresses that look like 2001:db8::10. They work the same way, with far more addresses. This course sticks to IPv4, because it’s still what you’ll see first.

in one sentence IPv6 is the newer version with far more addresses; the ideas are the same.

Servers need a home

When you watch a video or open a website, your message goes to a server: a computer whose job is to answer other computers. A video server sends videos; a web server sends web pages. Servers never sleep: someone, somewhere, always wants a video.

Think of a giant hotel, but for computers instead of people. Every room is full of servers, the lights never go off, and there’s staff on duty day and night. That’s a data center: a building (or a big room in one) full of servers, with everything they need to keep running all the time.

in one sentence A data center is a building full of servers, kept running day and night.

What’s inside

• Racks: tall metal cupboards, about two metres high, with servers stacked inside like books on shelves. Rack space is measured in U (one unit, 4.45 cm); a normal rack holds 42U.
• Power: lots of it, from two separate supplies, plus big batteries and diesel generators, so the servers keep running even if the electricity goes off.
• Cooling: servers get hot, like a laptop on your knees, times thousands. Rows of racks face each other so cold air goes in at the front and hot air comes out of the back, where it’s pulled away.
• Security: guards, cameras, locked cages and ID cards. You can’t just walk in.
• The network: switches and routers (from lesson 2.1) joining every server to every other server, and to the internet.

in one sentence Inside: racks of servers, lots of power, lots of cooling, tight security, and a network joining it all.

Two of everything

What happens if a car gets a flat tyre? You use the spare. Data centers have a spare for everything: two power supplies, two cables, two switches, two internet connections. Having a spare ready like this is called redundancy. If one thing breaks, the other takes over and nobody notices.

in one sentence Redundancy means a spare for everything, so one broken part doesn’t stop anything.

Leaf and spine

Think of a school with long corridors of classrooms, and two main staircases. Every corridor joins both staircases. To get from any classroom to any other, you walk along your corridor, up or down a staircase, and along the other corridor: always the same short trip. And if one staircase is closed, you use the other.

Most data center networks are built just like that, and it’s called leaf-spine:

• a leaf switch sits at the top of each rack, and that rack’s servers plug into it (people also call it a top-of-rack switch). The leaves are the corridors.
• the spine switches sit in the middle. They’re the staircases.
• Every leaf connects to every spine. So any server reaches any other by going leaf → spine → leaf.

in one sentence In leaf-spine, every leaf connects to every spine, so any two servers are always leaf → spine → leaf apart.

The door and the guard

Where the internet comes into the building, there’s a router (the post office from lesson 2.1) called the edge router, because it sits at the edge. Next to it is a firewall: a security guard for messages. It checks every message coming in and only lets through the kinds that are allowed, like requests for a video.

in one sentence The edge router brings the internet in, and the firewall checks every message at the door.

North-south and east-west

Draw the data center like a map, with the internet at the top:

• messages going in and out, between the internet and the servers, travel up and down the map. They’re called north-south traffic.
• messages between servers inside the data center, like a web server asking a database (a server that stores information, like a giant filing cabinet) for something, travel across the map. That’s east-west traffic.

And the cloud? It’s just someone else’s data center. When a company uses the cloud, it rents servers in a data center that a cloud company runs.

in one sentence North-south is traffic in and out of the data center; east-west is servers talking to each other inside it.

In lesson 2.1 you saw two things happen:

1. On a hub, two PCs talking at the same time collided, and both messages were ruined.
2. A message to everyone went to every host, until it reached a router, which stopped it.

This lesson gives each of those a name, so you can draw them on any network. First, one word: a domain is the area something reaches, like a kingdom is the land a king rules.

in one sentence A domain is the area something reaches; this lesson is about two of them.

Collisions: talking at the same time

Think of a classroom with one rule: only one person can speak at a time. If two people start at once, nobody understands either of them, so both stop, wait a moment and try again.

On a network, when two hosts send at the same moment on a shared line, their signals crash into each other. That’s a collision. A collision domain is the group of devices that can collide with each other: the whole classroom.

• A hub shares one line between everything plugged into it, so the whole hub is one collision domain.
• A switch gives each port its own private line, so every switch port is its own collision domain. (Modern switch ports can even send and receive at the same moment, like a road with a lane each way. That’s called full duplex, and it means no collisions at all. The port is still where the boundary goes.)
• A router port is a boundary too.

in one sentence A collision domain is everyone who has to take turns on one shared line: a whole hub, but just one port on a switch.

Broadcasts: an announcement to everyone

Think of the speakers in a school’s corridors. An announcement reaches every classroom in the building, but not the school down the road.

On a network, a message to everyone is called a broadcast. It has a special to address that means all of you: the MAC address FF:FF:FF:FF:FF:FF. A broadcast domain is the group of hosts that all hear each other’s broadcasts: the whole school building.

• Hubs and switches pass broadcasts to everyone, so they don’t split broadcast domains.
• A router stops broadcasts, like the school gate. Every router port is the edge of a broadcast domain.

in one sentence A broadcast domain is everyone who hears a message sent to all; only a router stops it.

The whole picture

Why care? Collisions slow a network down, because everyone waits. Broadcasts make every host stop and read them, so a broadcast domain with thousands of hosts is like a school with announcements every few seconds. Network engineers keep broadcast domains small on purpose.

in one sentence Switches and routers split collision domains; only routers split broadcast domains.

The apartment building’s mail room

Two computers can share one cable. But plug in three, or thirty, and you need a box in the middle that everyone’s cable plugs into, to pass messages between them.

Think of a big apartment building, and three different ways the post could get from flat to flat:

• The loudspeaker. Whoever has a letter reads it out over a loudspeaker in every flat. Everyone hears everything, and only one person can use the loudspeaker at a time.
• The mail room clerk. A clerk who knows which flat everyone lives in takes each letter straight to the right door.
• The post office. For a letter to another town, the clerk hands it to the post office, which knows the way to every town.

Those are the three boxes in this lesson: the hub, the switch and the router.

in one sentence A hub shouts everything to everyone, a switch delivers to the right door, and a router sends things to other networks.

Ports

A port on one of these boxes is a socket where a cable plugs in, like the numbered mail slots in a mail room. Ports are numbered: port 1, port 2, and so on. (This is a different thing from the port numbers in lesson 1.2: same word, different meaning. Networking does that sometimes, sorry.)

in one sentence A port is a numbered socket on a box, where a cable plugs in.

The hub: the loudspeaker

A hub doesn’t read anything. Whatever signal comes in on one port, it repeats straight out of every other port. That makes it a layer 1 box: it only deals with the signal itself.

Because everyone shares, only one host can talk at a time. If two talk at once, their signals crash into each other and both are ruined. That’s called a collision, and both have to try again.

Hubs are very old: you’d only find one in a museum now. But they’re the easiest way to see why switches were invented.

in one sentence A hub repeats every message to everyone, so only one host can talk at a time.

The switch: the mail room clerk

A switch reads the MAC address on every frame (remember from lesson 1.2: a frame is a message in its layer 2 envelope, with a to and from MAC address on it). That makes it a layer 2 box.

It keeps a list called the MAC address table: which MAC address lives on which port. Nobody types it in; the switch fills it in by itself. Every frame says who it’s from, so when one arrives, the switch notes that MAC address is on this port.

Then it sends each frame only out of the port where the to address lives. Sending a message on like this is called forwarding it. If the switch doesn’t know where the to address is yet, it sends a copy out of every port, just this once. That’s called flooding.

in one sentence A switch learns which MAC address is on which port, and sends each frame only where it needs to go.

The router: the post office

A switch joins hosts on one network. To reach a host on a different network, like your home reaching a website, you need a router.

A router reads a different address: the IP address. You’ll learn all about IP addresses in lesson 3.1; for now, think of it as a postal address with a town in it. The first part of an IP address says which network (which town) a host is in. That makes the router a layer 3 box.

A router has a foot in each network it joins, and keeps a routing table: a list of networks and which way to send messages for each one, like the post office knowing which van goes to which town.

in one sentence A router joins different networks, and reads the IP address to pick the way to the right one.

To keep the pictures readable, the MAC addresses below are shortened to one letter: PC A’s MAC is just A. (A PC, personal computer, is any laptop or desktop computer.)

Sending a birthday present

Think about posting a birthday present to a friend in another town. Lots of people help, and each does one job:

1. You choose the present and write the card.
2. You wrap it, so nobody can peek.
3. You write part 1 of 2 on the box, because there’s a second parcel coming.
4. You write for Sam and ask for recorded delivery, so you’ll know it arrived.
5. You write the full street address: town, street, house number.
6. The post office puts a label on it for the local van: next stop, the sorting office.
7. The van drives it down the road.

Nobody needs to know how the others do their job. The van driver doesn’t care what the present is; you don’t care which road the van takes.

in one sentence Sending something is lots of small jobs, and each helper only does their own.

Layers

Networks work the same way. Sending one message takes lots of jobs, so they’re split into layers. A layer is one job in a stack of jobs, like one floor of a building. Each layer only talks to the floor just above it and the floor just below it.

The most famous way of splitting the jobs is called the OSI model. A model is a simple picture of how something works, the way a map is a simple picture of a town. OSI stands for Open Systems Interconnection, which means a shared plan so computers made by different companies can connect. It has seven layers, and network engineers use the numbers all the time: “it’s a layer 1 problem” means check the cable.

in one sentence The OSI model splits networking into seven layers, each with one job.

The seven layers

Here they are, matched to the birthday present. Some words in the last column are new; each one gets its own lesson later, so just notice where it sits for now.

A port number is a number that says which program on a computer a message is for, like the name of the person in a house who should open the parcel. Lesson 5.1 covers ports, and the common numbers, in full.

Remember the layers from the bottom up with Please Do Not Throw Sausage Pizza Away: Physical, Data link, Network, Transport, Session, Presentation, Application.

in one sentence From the bottom: Physical, Data link, Network, Transport, Session, Presentation, Application.

Wrapping and unwrapping

On the way out of a computer, a message starts at the top, layer 7, and goes down. Each layer sticks a small label on the front, called a header, with the information that layer needs, like the address label on a parcel. Putting a message inside another layer’s wrapping like this is called encapsulation (it means putting into a capsule).

At the bottom, layer 1 sends it as bits. A bit is the smallest piece of computer data there is: a 1 or a 0, like a light switch that’s on or off. Down a cable, bits are pulses of electricity.

On the way in to the other computer, it goes up the layers, and each layer reads its own label and takes it off, until layer 7 has the original message.

The message has a different name depending on where it is: segment at layer 4, packet at layer 3, frame at layer 2 (a frame is a packet in its local-delivery envelope), and bits at layer 1. They’re all the same message, just wrapped differently.

in one sentence Going down, every layer adds a label; going up the other side, every layer takes its label off.

One honest note

The internet doesn’t really use all seven layers separately. It uses a simpler plan called TCP/IP (named after two of its rules: Transmission Control Protocol and Internet Protocol; a protocol is just an agreed set of rules, like everyone agreeing to write the address on the front of an envelope). TCP/IP squashes layers 5, 6 and 7 into one. People still use the OSI numbers when they talk, so learn them; just don’t be surprised when, in the DHCP lesson, layer 7 comes straight after layer 4.

in one sentence The real internet squashes layers 5 to 7 together, but everyone still talks in OSI layer numbers.

This whole course starts from nothing. Every word gets explained before we use it, starting with the everyday ones.

Passing a note in class

Imagine you want to tell a friend on the other side of the classroom something. You write it on a scrap of paper, fold it, write their name on the outside, and pass it along the row. Each person in between looks at the name and passes it on, until your friend gets it, opens it and reads it.

That’s everything a computer network does. The rest of this course is just the details.

in one sentence Sending something over a network is like passing a named note across a classroom.

Data and messages

Data is any information a computer keeps or sends: words, photos, songs, videos, the moves in a game. Inside the computer, all of it is stored as numbers.

A message is some data sent from one place to another, like your note.

in one sentence Data is information; a message is some data on its way somewhere.

What’s a network?

A network is a group of things joined together so they can send each other messages. The classroom is a network: people who write notes, and a way of passing them along.

Computers join up in two ways:

• with a cable: a wire with a plug on each end. Electricity travels along it, carrying the message. The usual network cable is called an Ethernet cable. Ethernet is just the name of the most common kind of wired network; the plug looks like a chunky phone plug.
• with Wi-Fi: the same idea, but using radio waves, invisible signals through the air, like a walkie-talkie. No cable needed.

Two laptops joined by one cable are a network. So is the whole internet: millions of networks all joined together, all over the world.

in one sentence A network is things joined by cables or Wi-Fi so they can send each other messages.

Hosts: the ones writing and reading

In the classroom, some people write and read notes: you and your friend. Others just pass them along.

On a network, the things that send and receive their own messages are called hosts. Your laptop is a host. So is a phone, a games console, a smart TV, a printer, and the big computers far away that send you videos and web pages.

The boxes in between that only pass messages along are called network devices. You’ll meet them in chapter 2.

in one sentence A host is anything that sends or receives its own messages, like a laptop or a phone.

Addresses: who is it for?

On the classroom note, you wrote your friend’s name on the outside so everyone knew who it was for. On an envelope you write an address. An address is just the thing that says who, or where, a message is going to.

Every host on a network needs one too, or nobody would know who a message is for.

in one sentence An address says who a message is for, like the name on the outside of a note.

The network card and its MAC address

The part of a host that actually sends and receives messages is its network card. It’s the bit the cable plugs into, or the little radio that does Wi-Fi. (People also call it a NIC, short for network interface card: interface means the place where two things meet, here the computer and the network.) A laptop usually has two: one for Wi-Fi, one for a cable.

Every network card is given a name at the factory, and it keeps it for life. That name is its MAC address. (MAC stands for Media Access Control, a technical way of saying the card’s own address on the network. It’s nothing to do with Apple Mac computers: every network card has one.) It looks like this:

AA:AA:AA:11:11:11
3C:22:FB:9A:04:7E

Six pairs of characters, with colons between them. They use hexadecimal (hex for short), a way of counting with sixteen digits instead of ten: 0 to 9, then A to F, so A means ten and F means fifteen. Don’t worry about the maths: just know it’s a name made of numbers and letters. The first half usually says which company made the card; the second half is that card’s own number.

in one sentence Every network card has a MAC address: a name it got at the factory, like AA:AA:AA:11:11:11.

Packets: lots of small notes

What if your note is really long, like a whole story? You might tear it into numbered postcards and send them one at a time. Then other people can pass their notes in between, and if one postcard gets lost, you only rewrite that one.

Computers do exactly this. They chop a big message into small pieces called packets. Every packet has a to address and a from address written on it, like an envelope.

in one sentence Big messages are chopped into small packets, and each packet carries a to and a from address.