Topic 0.1
What a Network Is: Hosts, Links, Packets
In one line
A network is devices (hosts) connected by links that exchange small chunks of data called packets; the internet is millions of such networks connected to each other.
Think of it like this
The postal system. You don't ship a whole book in one envelope; you split it into numbered pages, each in an envelope with a from and to address. Envelopes travel through sorting offices (routers), possibly by different routes, and the receiver puts the pages back in order. That's packet switching.
Key ideas
- 01
A HOST is any device that sends or receives data: your laptop, a server, a phone, a container. A LINK is whatever connects them: Ethernet cable, Wi-Fi, fibre. Devices that forward traffic between links are SWITCHES (within one local network) and ROUTERS (between networks).
- 02
Data is sent as PACKETS: typically up to ~1,500 bytes each on Ethernet (the MTU, maximum transmission unit). A 2 MB image becomes roughly 1,400 packets. Each packet carries headers with addressing and control information plus a slice of the payload.
- 03
PACKET SWITCHING (the internet's design) lets many conversations share the same links, with each packet routed independently. Its alternative, circuit switching (old telephone networks), reserved a dedicated path for each call.
- 04
The INTERNET is a 'network of networks': your home network, your ISP, backbone carriers, cloud providers, and content networks, connected at exchange points and bound together by shared protocols (IP, TCP, DNS...). No single organisation runs it.
- 05
Key vocabulary for the whole course: CLIENT (initiates), SERVER (listens and responds), PROTOCOL (the agreed rules of a conversation), PORT (which program on a host), and IP ADDRESS (which host).
Code & diagrams
One request crossing several independent networks.
# Which hops does traffic take to a website? (Linux/macOS; Windows: tracert)
traceroute -n example.com
# The MTU of your network interface (usually 1500)
ip link show | grep mtu # Linux
ifconfig | grep mtu # macOSExplain it without notes
Why does the internet split data into packets instead of sending each file as one continuous stream?
What's the difference between a switch and a router, in one sentence each?
Practice
Run traceroute (or tracert) to a website you use. How many hops are there, and can you tell roughly where your ISP ends?
If a 1 MB file is sent over Ethernet with a 1,500-byte MTU, and each packet spends 40 bytes on IP and TCP headers, approximately how many packets are needed?
Trade-offs
- ↔
Packet switching makes networks efficient and resilient but gives no guarantees: packets can be delayed, lost, duplicated, or reordered. Every reliability feature you'll meet (TCP retransmission, retries, timeouts) exists to cope with that.
Done when you can
I can explain packets, hosts, links, switches, and routers.
I know what MTU means and its usual Ethernet value.
I can run traceroute and interpret the hops.