What Is Firewall?

A firewall is a security system that sits between your internal network and the outside world — typically the internet — and controls which traffic is allowed in and out. Think of it as a security checkpoint: every data packet entering or leaving your network is inspected against a set of rules, and only traffic that meets your security criteria is permitted through. Everything else is blocked.

Firewalls come in two primary forms: hardware firewalls (dedicated physical appliances that sit between your internet connection and your network) and software firewalls (applications running on individual computers or servers). Most businesses use both — a hardware firewall at the network perimeter and software firewalls on individual devices. Modern firewalls, often called next-generation firewalls (NGFWs), go beyond basic packet filtering to include deep packet inspection, intrusion prevention, application awareness, and threat intelligence feeds that identify and block known malicious traffic in real time.

For small and mid-sized businesses, proper firewall configuration is one of the most impactful security investments you can make. A well-configured firewall prevents unauthorized access to your network, blocks known malicious IP addresses and domains, controls which applications can communicate externally, and provides logging that helps identify suspicious activity. However, a firewall is only as good as its configuration and maintenance — default settings, outdated firmware, and overly permissive rules are common weaknesses that attackers exploit.