What Is a Firewall?
A firewall inspects network traffic and allows or blocks it according to rules. It is the classic enforcement point of a network boundary — deciding what may cross between a trusted network and an untrusted one. Firewalls are foundational, but they are one layer, not a fortress: knowing what they can and cannot do keeps expectations honest.
What kinds of firewall are there?#
Firewalls have grown more capable over time:
| Type | Inspects | Notes |
|---|---|---|
| Packet filter | IP, port, protocol | Fast, stateless |
| Stateful | Connection state | Tracks sessions; the common baseline |
| Application / proxy | Application-layer content | Deeper inspection |
| Next-gen (NGFW) | App awareness, IPS, threat intel | Modern all-in-one |
| WAF | HTTP application traffic | Specialized for web apps |
The move from packet filter to next-gen is a move up the OSI layers: from “which port?” to “which application, doing what?”
What can’t a firewall do?#
Plenty. A firewall filters traffic it sees at the boundary — it does nothing against a malicious insider, an already-compromised host, encrypted traffic it cannot inspect, or attacks that ride on allowed ports (most web attacks arrive over permitted 443). It is a control on where traffic may flow, not a guarantee that allowed traffic is safe.
Firewalls are the enforcement layer of network design. More at the Network Security hub.
Frequently asked questions#
What does a firewall do?
A firewall inspects network traffic and allows or blocks it according to configured rules — by IP address, port, protocol, and, in more advanced firewalls, application and content. It enforces a boundary between networks of different trust levels, such as the internet and an internal network.
What is the difference between a network and a host firewall?
A network firewall sits at a boundary (like the edge of a network) and filters traffic between segments. A host firewall runs on an individual device and controls that machine’s own traffic. Defense in depth uses both: the network firewall for perimeter policy, host firewalls to protect each endpoint.