Firewall

A firewall is a network security system designed to protect a trusted private network from unauthorised access or traffic coming from an untrusted outside network — for example, the Internet, or even different sections of the same network to which the private network is connected. Think of it as a guarded gate between two worlds: everything that tries to cross from the untrusted side into the trusted side must pass through this gate and be checked.

The firewall can be implemented in software, in hardware, or in both. Software firewalls run as programs on individual computers or servers, while hardware firewalls are dedicated physical devices placed between networks. Many organisations use a combination of both for stronger protection.

A firewall matters because of the way malware spreads. A worm, for instance, has the capability to move across networks and infect other computers on its own. The firewall acts as the first barrier against such malware — it stands between your network and the outside world, blocking malicious traffic before it can reach your machines.

A firewall acts as a network filter. Based on predefined security rules, it continuously monitors and controls both incoming and outgoing traffic. Every packet of data that tries to enter or leave the network is examined against these rules, and only traffic that matches the allowed conditions is permitted to pass.

Here is a concrete example of how a rule works. In a school LAN, a rule can be set in the firewall so that a student cannot access data from the finance server, while the school accountant can access the finance server. The firewall enforces this distinction automatically — it does not rely on human judgment at the moment of access, but on the rules that were configured in advance.

So the key ideas to carry forward are:

• Purpose: protect a trusted private network from unauthorised access or traffic from an untrusted network. …