If your network is to be kept secure it is often essential that some filtering takes place.

For example, some staff wish to work from home with their laptops and they need to access files from within the company network.

In this case a filter would be set up that accepts data packets coming from that particular laptop. Other filtering rules would block unwanted packets trying to come in.

Just like Gateways, a Filter can be a mix of hardware and software components.

Note that a filter can also prevent data packets from leaving the company network. For example, a rule could be set up that only allows an authorised server within the network to send data outside the local network.

A filter is an essential component of a 'Firewall'.