4.1 The Internet

What you need to know

• A computer network is a group of interconnected computing devices that can send and receive data. The internet is a network of networks connected by shared standards — no single organization owns or controls it.
• Routing is finding a path from sender to receiver. Data can take many different paths; routers pass packets toward their destination.
• Data is sent in packets: small chunks that each contain the data plus metadata (source and destination addresses, sequence number). Packets from one message may travel different routes and arrive out of order; they're reassembled at the destination using the sequence numbers.
• Every device has an IP address — a unique numeric identifier used for routing. IPv4 uses 32 bits (about 4 billion addresses, now exhausted); IPv6 uses 128 bits.
• Protocols are agreed rules that specify how data is formatted, addressed, transmitted, and received. Because they're open standards, any device following them can communicate with any other.
• Key protocols: IP (addressing and routing packets), TCP (reliable delivery — checks all packets arrive, requests re-sends, reorders), UDP (faster, no delivery guarantee — used for streaming), HTTP/HTTPS (requesting and sending web pages; HTTPS adds encryption).
• DNS (Domain Name System) translates human-readable names like example.com into IP addresses.
• Bandwidth is the maximum data-transfer rate of a connection, measured in bits per second. Higher bandwidth = more data per second.
• The World Wide Web is a system of linked pages and resources that runs on the internet. They aren't the same thing: the internet is the network; the web is one service using it.
• The internet's scalability — its ability to keep working as it grows — comes from its layered, standardized, decentralized design.

Worked example

You type example.com and press Enter. Your browser asks a DNS server for that name's IP address. It then sends an HTTPS request. That request is split into packets, each stamped with your IP, the server's IP, and a sequence number. Routers forward each packet along whatever path is available — packet 3 might go through Denver while packet 4 goes through Dallas. The server's TCP layer collects them, puts them in order, and asks for any that are missing. Then it sends the page back the same way.

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• The internet is decentralized: no single authority controls it. It's a network of independently-operated networks that agree to talk using common protocols. That's why it can't easily be "turned off" and why it scales — anyone can add a network.
• Packets carry a chunk of data plus metadata: source address, destination address, and a sequence number. The sequence number is what lets the receiver reassemble packets that arrive out of order. If a packet is missing, TCP asks for it again.
• Routing happens hop by hop. Each router looks at a packet's destination and forwards it toward the next router closer to that destination. No router knows the whole path; each knows the next step. Different packets of the same message can take different paths.
• IP handles addressing and routing — getting packets to the right machine. TCP sits on top and handles reliability — making sure they all arrive, in order, and asking for re-sends. UDP is TCP's faster, unreliable sibling used when speed matters more than perfection (live video, games).
• HTTP is the protocol web browsers and servers use to request and send pages. HTTPS is HTTP with encryption (via TLS), so intercepted traffic can't be read. The padlock icon means HTTPS.
• DNS is the internet's phone book: it maps names like collegeboard.org to IP addresses. Without DNS you'd type numbers. DNS itself is a distributed, hierarchical system — another example of scalability.
• IPv4 addresses are 32 bits (four numbers 0–255, like 192.168.1.1) — about 4.3 billion, which ran out. IPv6 uses 128 bits — enough for every device imaginable. The transition is ongoing.
• Bandwidth is capacity (bits per second); latency is delay (how long one packet takes to arrive). A satellite link can have high bandwidth and high latency. The CED focuses on bandwidth.
• The World Wide Web is one application built on the internet, alongside email, file transfer, streaming, and games. The web = HTTP + HTML + browsers. The internet = the underlying network. Different things.
• Open standards (the protocols are public, not owned) are why the internet works across every manufacturer and country. Anyone can implement TCP/IP; nobody needs permission.

Mistakes that cost points

• Using internet and web interchangeably. The exam tests the distinction. Web is a service; internet is the network.
• Claiming packets take the same path or arrive in order. Neither is guaranteed. Sequence numbers and reassembly handle it.
• Assigning the wrong job to a protocol. IP = addressing/routing. TCP = reliable delivery. HTTP = web pages. DNS = names to addresses. Learn the four.
• Thinking a router knows the whole route. It knows the next hop. That's what makes rerouting around failures automatic.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.