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Arriving and Climbing

Lesson 31 of 32 in Coddy's The OSI Model course.

At the server, the journey reverses: de-encapsulation. The NIC checks the frame's MAC, layer 3 confirms the IP, and layer 4 reads the destination port to decide which program receives the data. Port 443 wakes the web server, port 22 the SSH daemon, port 53 the DNS service.

This port-based hand-off is called demultiplexing, and it is why one machine can host many services at once without them stealing each other's traffic.

From there the data climbs to layer 7, the request is parsed, and a response begins the same journey in reverse.

challenge icon

Challenge

Beginner

Packets arrive for three destination ports. Print the service each one wakes, one per line, uppercase:

  1. Port 443
  2. Port 22
  3. Port 53

Expected output:

HTTPS
SSH
DNS

Try it yourself

Terminal

All lessons in The OSI Model

1Why Layers

The Layering IdeaSeven LayersNumbers and NamesRecap - The Stack

4The Network Layer

IP and TTLPing and ICMPRouting TablesRecap - Layer Three

7Encapsulation

Wrapping DataHeaders EverywhereReading a CaptureRecap - Follow the Wrap

2The Physical Layer

Bits on a WireHow Long Will It TakeDuplex and Collisions

5Transport: TCP and UDP

PortsThe TCP HandshakeTCP vs UDPSockets in ActionRecap - Layer Four

8The Journey of a Packet

A Click HappensCrossing the NetworksArriving and ClimbingRecap - End to End

3Data Link and MACs

Frames and MACsReading a MACARPRecap - Layer Two

6The Upper Layers

Session and PresentationThe Application LayerWatching the LayersRecap - Top of the Stack

Practice on your own: Terminal playground