Why layers

Networking is split into layers so each part can change independently — you can replace copper with fibre without rewriting your web browser.

The seven-layer OSI model adds Session and Presentation between Transport and Application; in practice most engineers think in the five-layer TCP/IP model above.

Encapsulation

Each layer wraps the data from the layer above with its own header: the web page goes into a TCP segment, which goes into an IP packet, which goes into an Ethernet frame, which goes onto the wire as bits. The receiver unwraps in reverse.

Devices by layer

• Hub / repeater (1): repeats bits to every port — obsolete.
• Switch (2): forwards frames to the right port using MAC addresses.
• Router (3): forwards packets between networks using IP addresses.
• Firewall (3–7): filters traffic by address, port and increasingly by application.