Firewall
A network control that allows or blocks traffic based on rules (IP, port, protocol), enforcing boundaries between systems.
Definition
A firewall is a set of rules that allow or block network traffic based on properties like source/destination IP, port, and protocol.
Firewalls can exist at many layers: on a host, at the network edge, in cloud security groups, or inside a cluster network policy.
How it shows up in practice
- A service is healthy but unreachable from another network segment.
- Only some ports work (e.g., 443 allowed, 5432 blocked).
- Outbound access is restricted (common in production environments).
Firewalls are part of the runtime environment because they shape what connections are even possible.