One message, many steps
The destination is only part of the journey.
Once a device knows an IP (Internet Protocol) address, it still has work to do. An application needs to identify the service it wants, the message must be packaged for delivery, and each router must choose where to forward the next piece.
The useful model is layered: different parts of the network solve different parts of the delivery problem.
Local forwarding
A switch learns where devices are.
When a frame arrives, an Ethernet switch records the source MAC (Media Access Control) address and the port where it arrived. This builds a forwarding table over time instead of requiring the switch to know every device in advance.
For a known destination MAC address, the switch forwards the frame only through the matching port. For an unknown destination, it floods the frame to the other ports in the same local network so the destination can answer. It then learns from that reply.
Build the model
Four jobs on the route
Packet
Breaks a larger exchange into manageable units that can be addressed and forwarded.
Router
Reads enough destination information to choose the next network hop.
Port
Identifies the service or conversation that should receive traffic on a device.
Protocol
Defines how systems format, sequence, acknowledge, and interpret the exchange.
Mini experiment
Trace a packet
Choose a service and follow the simplified decisions made as the packet moves toward it.
Ready. Choose a service, then trace its packet.
Knowledge check
Test your journey model
Answer seven questions. The answer order changes each time.