The OSI Model, Just Enough to Be Useful

The OSI model splits networking into 7 layers, from the physical cable (layer 1) up to the application a user actually sees (layer 7). You don't need to memorize all 7 to be useful, but three come up constantly in security work:

• Layer 3, Network — IP addresses and routing live here. This is where firewalls make most of their allow/block decisions.
• Layer 4, Transport — TCP and UDP live here, deciding how reliably data gets delivered (more on this in the next article).
• Layer 7, Application — HTTP, DNS, email, the actual protocols apps use. Most of the vulnerabilities in the Web Application Security subject live here.

When a tool or a job description says "layer 7 firewall" or "layer 3 switch," this is the vocabulary it's drawing from.