Network models break communication into layers so that each layer does one job and relies on the one below it. Knowing the layers helps you place protocols, devices and attacks, and security questions often ask where something operates. The Open Systems Interconnection (OSI) model has seven layers: 1 Physical (cables, radio, signals), 2 Data Link (frames, MAC addresses, switches, ARP, VLANs), 3 Network (IP addressing and routing, routers, ICMP, IPsec), 4 Transport (TCP and UDP, ports), 5 Session (setting up and managing sessions), 6 Presentation (data formatting, encoding, and conceptually encryption), and 7 Application (protocols users' software speaks, such as HTTP, DNS and SMTP). A common memory aid from layer 1 up is Please Do Not Throw Sausage Pizza Away.
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