Packets, routers and the path your data takes

The last lesson followed one click from your Mac to a website. This lesson slows down the “connecting” step: what a packet is, how routers decide where to send it, and how you can watch that path yourself.

Your message, cut into pieces

Nothing you send over the internet travels in one piece. As Cloudflare’s Learning Center explains, data is broken into small segments called packets before it leaves your device, then reassembled by whoever receives it. This is called packet switching: because packets from many different computers can share the same wires and equipment, billions of devices can exchange data on the same networks at once, each packet forwarded independently and not necessarily arriving in the same order it was sent.

Routers: a decision at every hop

A packet does not travel directly to its destination. It is handed from one router to the next, and each router makes its own choice about where to send it next. Cloudflare describes routers as reading a packet’s header — the part that names its destination — and checking that against a routing table, similar to a train conductor checking a ticket to decide which platform a passenger belongs on. Routing tables can be set manually (static routing) or updated automatically as conditions change (dynamic routing), the way a mapping app reroutes around traffic. A router does this millions of times a second, for millions of packets, from many different senders at once.

Because the internet is a network of many independently run networks, a single connection to a website usually crosses several of them: your home or office network, your ISP, one or more transit networks, and finally the network that hosts the site. Nobody owns the whole path, and no single company sees all of it.

Watching the path yourself: traceroute

macOS ships with a tool that shows this hop-by-hop journey. Cloudflare’s Learning Center explains that traceroute works by sending packets with a gradually increasing time-to-live value, so that each router along the way sends back a small error message revealing itself, one hop at a time. The result is a list of the routers your packet passed through and how long each hop took.

$ traceroute 1.1.1.1

A real trace to Cloudflare’s 1.1.1.1, shown in Cloudflare’s own documentation, looks something like the router in your home, then a hop or two inside your ISP, then one or more networks you have never heard of, before finally reaching the destination. Some hops show three stars (* * *) instead of a time: that usually means a router is configured not to reply to traceroute, not that anything is broken.

Crossing oceans: the cables you never see

For a connection between continents, the packets very likely travel through a submarine fibre-optic cable at some point. The International Cable Protection Committee, an organisation of governments and cable owners founded in 1958, notes that these cables carry international voice and data traffic more cheaply and reliably than satellites: a call that would need to travel tens of thousands of kilometres up to a satellite and back can instead go point to point along the sea floor. A single fibre pair in one of these cables can now carry a digitised capacity described by the ICPC as equivalent to 150,000,000 simultaneous phone calls, and each cable typically bundles several such fibre pairs. Because a fault or a ship’s anchor can cut a cable, the industry relies on having several independent routes rather than a single line between any two regions.