No router knows the whole route to anywhere. Each one only knows the next step. And because there’s usually more than one way through, cutting a wire slows the internet down instead of breaking it.
No reading — this page is the reading. Bring 6.4 with you: every packet carries its destination address, and that’s all a router needs.
Internet. Packets are addressed (6.3) and numbered (6.4). Today: how they find their way — and what happens when part of the way is gone.
The card still got through. Nobody had a map. How?
Routing is finding a path from sender to receiver. On the internet it happens one hop at a time: each router looks at a packet’s destination address and sends it to the neighbor that’s closest to that destination. That’s it. No router knows the whole path, and no central computer plans it.
Routers keep telling each other which destinations they can reach, so when a link goes down, the neighbors notice and start sending traffic another way. The path is decided as the packet travels, and it can change from one packet to the next.
Historical note, one minute’s worth: the internet’s ancestor was designed in the 1960s–70s with survivability in mind — a network that keeps working when pieces of it are destroyed. That goal came before efficiency.
Send from A to H. The green path is the current route. Click any link to cut it (click again to repair). How many cuts before A and H can’t talk?
At a real computer, traceroute (Mac/Linux) or tracert (Windows) lists the hops between you and a server. Your teacher will run one live; in 6.12 you’ll measure your own.
Five questions. Sketch the network on paper for the ones with letters.
A city downtown may have dozens of fiber paths. A small island or a rural town may have one. When that one line is cut — by a ship’s anchor, a storm, a backhoe — everyone there goes offline together. Redundancy costs money, so it goes where the customers are.