TCP/IP vs OSI

Two models, one job

A network model splits the work of sending data into layers. Each layer has one job and hands off to the layer above or below it, so a Wi-Fi card doesn't need to know anything about web pages, and a web browser doesn't need to know anything about radio waves.

The OSI model (Open Systems Interconnection) has seven layers, numbered from the bottom: 1 Physical, 2 Data Link, 3 Network, 4 Transport, 5 Session, 6 Presentation and 7 Application. It was designed as a general reference, and it's the one most courses teach first.

The TCP/IP model is what the internet was actually built on. It's named after its two most famous protocols, TCP and IP.

How the four layers line up

The TCP/IP model has four layers. Each one covers one or more OSI layers:

• Link = OSI layers 1 and 2: the cables, Wi-Fi and the hop to the next device on your network.
• Internet = OSI layer 3: IP addresses and routing across networks.
• Transport = OSI layer 4: TCP and UDP, which carry data between programs.
• Application = OSI layers 5, 6 and 7 squeezed into one: the protocols apps use, like HTTP for the web and DNS for names.

So why learn OSI at all?

Because everyone still talks in its numbers. Engineers say "layer 2 switch", "layer 3 device" or "that's a layer 7 problem". A layer 7 problem means something is wrong with the app itself, not the cable, the Wi-Fi or the routing. A layer 1 problem is the physical stuff: an unplugged cable or a dead port.

Using the numbers is a quick way to say where to look. A good rule: OSI to talk about it, TCP/IP to run it.

Try it yourself

Next time a website won't load, work up the layers. Is the Wi-Fi connected (Link)? Do other websites load (Internet and Transport working)? If everything else works and only one app fails, it's probably a layer 7 problem: the app or the site, not your connection.

Good to know

• RFC 1122 defines the internet's link, IP (internet) and transport layers, and RFC 1123 covers the application layer.
• Some textbooks show a five-layer version of TCP/IP that splits Link into Physical and Data Link. Cisco's CCNA material uses both, so expect to see either.
• The Link layer is sometimes called "Network Access" or "Network Interface". Same layer, different name.

For CCNA learners

This sits under CCNA 200-301 exam domain 1.0, network fundamentals: know the seven OSI layers in order, which TCP/IP layer each one maps to, and use layer numbers when you describe devices and problems.