A network is just devices that can talk to each other. Your home has one, your school has one, and the internet links them all. These pictures show how a home network works, the OSI (Open Systems Interconnection) model that engineers use to describe any network in seven layers, and how every device gets an IP (Internet Protocol) address.

Your Home Network
Your Home NetworkA cut-away house. A line comes in from a pole to a modem, then to a router sending Wi-Fi waves to a laptop, a phone, and a TV, each tagged with its own address.YOUR HOME NETWORKModemRouter192.168.1.10192.168.1.11192.168.1.12From your ISPInside your home12345
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  1. 1The line from your provider. The connection reaches your home by fiber, cable, phone line, or a radio receiver on the roof. This is your link to the rest of the internet.
  2. 2Modem. The modem (short for modulator-demodulator), or a fiber or radio box, translates the signal from your provider into something your home network can use. Some homes have one box that is both a modem and a router.
  3. 3Router. The router is the traffic director for your home. It gives each device its own address on your home network, called an IP (Internet Protocol) address, and sends every packet to the right device.
  4. 4Wi-Fi. Wi-Fi is a short-range radio signal from your router, like a tiny radio station inside your home. It lets devices connect without cables. Walls and distance make it weaker, so a router near the middle of the home works best.
  5. 5Your devices. Phones, laptops, TVs, and printers all join the network. Each one gets its own address so the router knows who asked for what.
Seven Layers, Like Mailing a Package
Seven Layers, Like Mailing a PackageA stack of seven layers from Application at the top to Physical at the bottom, each matched with a step in mailing a package. Sending goes down the stack and receiving goes back up.SEVEN LAYERS, LIKE MAILING A PACKAGEApplicationThe apps you useWrite the letterPresentationFormats and locks dataTranslate and seal itSessionOpens and closes the talkStart the conversationTransportNumbers and checks piecesNumber the boxesNetworkAddresses and routesAddress and route itData LinkNext hop on local networkYour street's mail carrierPhysicalCables, light, radioThe road and the truckSending goes downReceiving goes up7654321
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  1. 7Application. This is the app you are using, like a web browser, email, or a video call. It is like writing the letter you want to send.
  2. 6Presentation. Data is put into a shared format both sides understand, and it can be locked with encryption so others cannot read it. It is like translating the letter and sealing the envelope.
  3. 5Session. Two devices open a conversation, keep it going, and close it when done. It is like starting and ending a phone call.
  4. 4Transport. Data is split into numbered pieces and checked so every piece arrives. TCP (Transmission Control Protocol) does this careful checking. It is like numbering boxes "1 of 3" and tracking each one.
  5. 3Network. Each piece gets an IP (Internet Protocol) address and a route between networks. Routers work here. It is like writing the street address and letting the post office choose the route.
  6. 2Data Link. Data moves one hop at a time across the local network using hardware addresses called MAC (Media Access Control) addresses. Switches and Wi-Fi work here. It is like the mail carrier on your street.
  7. 1Physical. The actual cables, pulses of light in fiber, or radio waves that carry 1s and 0s. It is the road and the truck.

Real networks mostly follow a simpler four-layer model called TCP/IP, named after its two main rules, TCP and IP. The seven OSI layers are still the most common way to learn networking and to talk about where a problem is, such as "it's a layer 1 problem" when a cable is unplugged.

IP Addresses Are Like Street Addresses
IP Addresses Are Like Street AddressesAn address card shows 192.168.1.10 as four numbers. An apartment building labeled with one public address holds three units with private addresses for a laptop, phone, and TV. Nearby houses each have their own public address.IP ADDRESSES ARE LIKE STREET ADDRESSESAn IPv4 address192.168.1.10four numbers, each 0 to 255203.0.113.25192.168.1.10Laptop192.168.1.11Phone192.168.1.12TVFront desk (DHCP)198.51.100.7198.51.100.8Your home: one public address, many private ones12345
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  1. 1Every device needs an address. To send a packet, the network needs to know where it is going. An IP (Internet Protocol) address is that address, like a street address on a mailbox.
  2. 2Four numbers. The common kind of address, IPv4 (Internet Protocol version 4), is written as four numbers separated by dots, such as 192.168.1.10. Each number is between 0 and 255.
  3. 3Private addresses inside. Inside your home, the router gives devices private addresses, often starting with 192.168 or 10. Millions of homes reuse these same private numbers, and that is fine because they never leave the home.
  4. 4One public address. Your router shares one public address with the whole internet, like an apartment building with one street address and many apartment numbers inside. This sharing is called network address translation (NAT).
  5. 5The front desk. Devices get their address automatically from the router through a service called DHCP (Dynamic Host Configuration Protocol), like a front desk handing out room numbers when guests arrive.
Running Out of Numbers: IPv4 and IPv6
Running Out of Numbers: IPv4 and IPv6A jar of marbles labeled IPv4 about 4.3 billion addresses, a SOLD OUT sign from 2015, a sky full of dots labeled IPv6 340 undecillion, and a two-lane sign showing IPv4 and IPv6 running together.RUNNING OUT OF NUMBERSIPv4: about 4.3 billion192.0.2.1SOLD OUTin 2015IPv6: 340 undecillion2001:db8::1IPv4 laneIPv6 lane1234
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  1. 1IPv4. IPv4 dates to 1983. Its addresses are 32 bits long (a bit is a single 1 or 0), which allows about 4.3 billion addresses. That sounded like plenty before phones, TVs, and doorbells all went online.
  2. 2Sold out. There are now more connected devices than IPv4 addresses. In North America, the American Registry for Internet Numbers (ARIN), which hands out addresses, gave out its last free IPv4 addresses on September 24, 2015. Sharing tricks like network address translation (NAT) help stretch the supply.
  3. 3IPv6. IPv6 (Internet Protocol version 6) addresses are 128 bits long and use numbers and letters separated by colons, such as 2001:db8::1. There are about 340 undecillion of them: 340 followed by 36 zeros. That is enough for every grain of sand many times over.
  4. 4Both at once. Most networks today run IPv4 and IPv6 side by side, like a road with two lanes. Your devices use whichever one works best, and you usually never notice.

Remember

Your router directs traffic and hands out addresses

Seven layers, from the app down to the wire

192.0.2.10

An IP address is a device's street address

Try it

1. Which OSI layer is the cable or Wi-Fi signal itself?

2. Why can millions of homes all use 192.168.1.10?

3. What hands out addresses to devices on your home network?

Learn more

References

  1. American Registry for Internet Numbers. (2015, September 24). ARIN IPv4 free pool reaches zero. https://www.arin.net/vault/announcements/20150924/
  2. Cloudflare. (n.d.-a). What is my IP address? Cloudflare Learning Center. https://www.cloudflare.com/learning/dns/glossary/what-is-my-ip-address/
  3. Cloudflare. (n.d.-b). What is the OSI model? Cloudflare Learning Center. https://www.cloudflare.com/learning/ddos/glossary/open-systems-interconnection-model-osi/
  4. Code.org. (n.d.). How the Internet works [Video series]. https://code.org/videos
  5. Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. J., & Lear, E. (1996). Address allocation for private internets (RFC 1918). RFC Editor. https://www.rfc-editor.org/rfc/rfc1918

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