Data is usually split into 'packets' before it is sent across a network.
This means that something like an image file is not sent as one single file. The image data is split into lots of different packets that can be reassembled to create the image file once they are received.
Firewalls are used to check data packets as they are sent to or received from a system or network.
A firewall can be used to:
Rules are created that take account of the following:
Firewalls protect against unauthorised access to a computer system by those who might want to steal data, cause malicious harm or take control of the system.