The OSI model, layer by layer
Seven layers from voltage to video call, each with the real hardware and real protocols that live there — and why engineers still count in these numbers.
The OSI model
Seven layers, agreed by committee in nineteen eighty four, and not one network on Earth implements them exactly. Yet every engineer still argues in these numbers, because they give you a shared vocabulary for saying where a problem lives.
Why layers at all
The point of layering is that each level only has to talk to its neighbours. Your video call has no idea whether it is riding on copper, glass or radio, and the radio has no idea it is carrying somebody's face.
One: Physical
Layer one is physical: voltages, pulses of light and radio waves. It has no notion of what it carries. A cut fibre, a bent pin or a cheap connector is a layer one fault, and it is remarkable how often that turns out to be the answer.
Two: Data link
Layer two is the data link, where bits become frames addressed to a forty eight bit hardware address burned into the card at the factory. This is where a switch lives, and where the network settles who is allowed to talk on a shared wire right now.
Three: Network
Layer three is the network, home of the internet protocol address and the router. Layer two can only reach machines on the same local segment. Layer three is what lets a packet hop across twenty separate networks to reach another continent.
Four: Transport
Layer four is transport, where TCP and UDP sit. TCP numbers every byte and resends whatever goes missing. UDP fires and forgets. Your video call chooses the careless one on purpose, because a late frame is worse than a lost one.
Five: Session
Layer five, the session layer, is meant to manage a conversation across many messages: opening it, checkpointing it, closing it politely. Almost nothing implements it as a layer of its own. The nearest thing you meet daily is the login cookie that lets a site remember you between clicks.
Six: Presentation
Layer six, presentation, is about form rather than meaning. Character encoding, compression and encryption belong here, which is conceptually where the S in HTTPS lives, even though the real implementation sits awkwardly across two layers.
Seven: Application
Layer seven is everything you can actually see: the web page, the email, the file transfer, the chat message. When somebody says a load balancer works at layer seven, they mean it can read the request and route on what it says.
Encapsulation
Data travels wrapped like a parcel. The application hands its message down, transport adds a header, the network adds another, the link layer adds one more plus a checksum at the tail. That is about fifty four bytes of wrapping before a single byte of yours leaves the machine, and each layer opens only the wrapper it put on.
Bits, frames, packets
Every layer renames the thing it carries. Bits at layer one, frames at layer two, packets at layer three, segments at layer four. All four words describe the same bytes wearing different numbers of coats.
Which box does what
Hardware maps onto this neatly. A hub is layer one and repeats blindly. A switch is layer two and learns hardware addresses. A router is layer three. A modern firewall reads all the way to layer seven, which is exactly why it costs so much more.
The four layer model
The model that actually runs the internet has four layers, not seven: link, internet, transport and application. It won because it shipped and worked. The seven layer version survives as the language everyone argues in afterwards.
Fault finding upward
Which turns all of this into a checklist. Is it plugged in, is the link light on, can you reach the gateway, does the name resolve, is the port open, does the reply make sense. Work upward and you find the break far faster than by guessing.
Watch it explain itself
Every step above is narrated aloud. Play it, or open it in the editor and make it yours — no account needed.