1 · Internet basics

Mostly short answers. Slides 1-1 → 1-41, 1-56 → 1-60.

What the Internet is

Nuts-and-bolts view: billions of hosts (end systems) running apps at the network edge, connected by links (fiber, copper, radio) and packet switches (routers, switches). Transmission rate = bandwidth.
Service view: infrastructure that provides services to apps (web, email, streaming) through a programming interface (sockets).
Protocol: defines the format and order of messages exchanged between entities, and the actions taken when a message is sent or received.

Access networks

How hosts connect to the first router.

Cable (HFC): every home in the neighborhood taps into the same coax cable, so bandwidth is shared. Asymmetric (download faster than upload).
DSL: each home uses its own existing phone line to the central office, so the line is dedicated.
Home network: modem + router/firewall/NAT + WiFi.
Wireless: WiFi (short range, inside a building) vs cellular 4G/5G (miles).
Enterprise and data center: Ethernet, high-speed links.

Physical media

Guided: twisted pair, coax, fiber. Fiber is fast, low-error, and immune to electromagnetic noise.
Unguided: radio (WiFi, cellular, satellite). Geostationary satellite has a big propagation delay (~270 ms).

Internet structure: a network of networks

Connecting every access ISP directly to every other takes O(N²) links. That doesn't scale.
So it's hierarchical: access ISPs pay regional ISPs, which pay Tier-1 global ISPs.
IXP (Internet Exchange Point): a place where ISPs meet to connect. Peering: two ISPs exchange traffic directly, usually for free.
Content provider networks (Google, Akamai) run their own networks to get close to users and skip the tier-1s.

Security

Packet sniffing: reading every packet on a broadcast medium (shared Ethernet, WiFi).
IP spoofing: sending a packet with a fake SOURCE address. Memory hook: spoof = lie about who you are.
DoS / DDoS: flooding a target with bogus traffic, often from a botnet of compromised hosts.
Defenses: authentication, encryption (confidentiality), integrity checks (digital signatures), firewalls.

Quick check

1. What 3 things does a protocol define?

The format and order of messages exchanged, and the actions taken when a message is sent or received.

2. Why is cable access "shared" but DSL isn't?

Cable: every home in the neighborhood is on the same coax, so they share its bandwidth. DSL: each home has its own phone line to the central office, so the line is dedicated.

3. Why don't access ISPs all connect directly to each other?

That's O(N²) connections, which doesn't scale. Instead ISPs connect hierarchically: access → regional → tier-1, and they peer at IXPs.

4. IP spoofing vs packet sniffing?

Sniffing = reading other people's packets on a shared medium. Spoofing = sending packets with a fake source address.

Next: 2 · Packet vs circuit switching →