An IP packet is a message in a bottle with an address on it. The sender never knows the full path — it only needs the first hop. From there, each router independently decides the next interface by longest-prefix matching the destination against its forwarding table. That per-hop decision is the entire architecture: no router knows the route, only a route.
Below IP sits the link layer. Packets cannot cross a wire alone — every hop encapsulates the packet in a frame addressed with MACs, and ARP is what translates IP to MAC one hop at a time. The TTL is the machine's anti-loop counter: every router decrements it, and a packet that cycles too long dies instead of circling forever. The journey's ordering matters because each layer trusts the one below: the frame dies at the next hop, the packet survives to the next router, and only the destination ever sees the payload.