Internet perimeter
Filter inbound/outbound.
What is a firewall? Filtering network traffic
A firewall is a security control that allows or blocks network traffic based on rules: addresses, ports, protocols, sometimes identity and application. It shrinks the attack surface between the Internet, DMZ, and internal networks. NGFWs add application inspection; it is not a substitute for patches or strong authentication.
In one sentence
A firewall decides which network traffic enters, exits, or flows—by rules.
Key points
- Explicit allow/deny rules; deny-by-default advised.
- Perimeter, cloud security groups, host firewalls.
- Complements: segmentation, IDS/IPS, WAF.
- Misconfigured, it creates a false sense of safety.
Term at a glance
- Firewall
- Network firewall · Network filter
- English term
- Firewall
- Domain
- Cybersecurity
- Category
- Network controls
- Level
- Beginner to intermediate
What does “firewall” mean exactly?
From hardware appliances to cloud security groups, the idea is the same: cut unused network paths.
NIST SP 800-41 covers architectures and policy practices.
For an SME, closing public admin ports and segmenting prod/dev already prevents many incidents.
How do you run a firewall effectively?
- 01
Map flows
Who must talk to whom, and why.
- 02
Minimal policy
Open only what is needed; document every exception.
- 03
Log and alert
Centralized logs, review abnormal denies.
- 04
Review regularly
Purge dead rules after each project.
Concrete firewall example
A manufacturing SME in Sherbrooke puts ERP behind a firewall: only the HTTPS reverse proxy is exposed. RDP is no longer on the Internet; access is via VPN. Opportunistic scans stop reaching the app server.
What is a firewall for?
Internal segmentation
Isolate OT, finance, guest Wi-Fi.
Cloud
Security groups / NSGs per environment.
Compliance
Demonstrable flow control.
Pros and cons of firewalls
- Shrinks attack surface
- Visible, auditable control
- Mature and well understood
- Foundation for segmentation
- Does not stop user phishing
- Complex rules become debt
- Bypass via poorly managed SaaS/VPN
- Insufficient alone against vulnerable apps
Network firewall vs WAF?
| Firewall | WAF (web application firewall) | |
|---|---|---|
| Layer | Network / transport (+ app NGFW) | HTTP/S application |
| Target | IPs, ports, protocols | Injections, bots, OWASP Top 10 |
| Placement | Perimeter, cloud SGs | In front of web sites/APIs |
| Complementarity | Yes—combine both | Yes—not a replacement |
Why firewalls matter for a Quebec SME
Many incidents start with a service exposed by mistake. A well-kept firewall is unglamorous but blocks ransomware paths and automated scans—especially on remote access.
FAQ
Does cloud still need firewalls?
Yes: security groups, WAFs, and egress policies remain critical.
Deny all outbound?
Often too strict; at least filter sensitive destinations and monitor.
Firewall = antivirus?
No. Different domains; they complement each other.
Who owns the rules?
Name an owner; orphaned rules become holes.
Related terms
Sources and references
Want a review of Internet exposure and segmentation? We can map flows and harden rules.
Talk about your perimeter