Firewalls

A firewall is a security system that separates a trusted network, such as a school's network, from an untrusted network such as the internet. It sits on the boundary and controls what traffic can pass between the two. It is a network security measure, not a physical lock on a door.

Data sent across a network is split into packets. The firewall monitors and checks each packet against a set of rules. A rule might allow only authorised computers, IP addresses or protocols to communicate, or allow data out but not in. If a packet matches an allowed rule, it passes; if it does not match, it is dropped. Dropping packets that fail the rules is called packet filtering.

What firewalls can block

• Malicious IP addresses: packets from a known harmful computer can be filtered out, so it cannot reach devices on your network.
• Ports: a firewall can prevent access to certain ports, which are numbered entry points for services. For example, a school could block the port used for remote desktop access from outside, while allowing the port used for secure web pages. This is port blocking.
• Websites: a firewall can block addresses of malicious or inappropriate websites, stopping users from opening them.

Hardware and software

A hardware firewall is a physical device, either a dedicated unit at the edge of a network or built into a router. It protects every device behind it. A dedicated unit costs more, because it is separate equipment that needs power, space and someone to set it up. A software firewall is installed on an individual computer. It is cheaper and travels with that device, but it uses the computer's processing power, so it can slow the computer down.