A firewall [I1] [I2] [I3] [I4] [I5] [F1] [S1] [S2] [S3] [S4] [S5] [E1] [E2] is basically a structure, a computer equipment, a software normally placed between Internet (or among Intranet) and the company local network in order to filter all packages and let enter only those that according to a certain security requirement are suitable to pass from the external net (thought to be less safe) and the internal net (thought to be safer).
![Un firewall](http://www.edurete.org/public/upload/firewall-01.jpg)
As you can see a firewall allows to link different net portions to different security levels. Therefore it can also be used inside a local net to make safe parts of local nets on which it is required a higher level of security.
![Firewall come muro taglia fuoco](http://www.edurete.org/public/upload/firewall-02.jpg)
But how does a firewall work? A firewall is a calculator with its own software. This calculator is linked from one side to a part of the net not very safe and from the other side on the net which needs to be made safe.
![Il funzionamento di un firewall](http://www.edurete.org/public/upload/funzionamento_firewall.jpg)
All packages pass through this software that has decisional processes. The processes follow strict rules previous decided, for example “ to reject all packages coming from a particular IP address (remember that inside an IP package there is always the origin address). Or I can decide to accept only some services like e-mail but to discard server web. The firewall is basically an intelligent element which divides one area from the other of the network.
It has some limits though:
- It does not protect from internal menaces and sabotage
- It does not protect from transmitting infected files or e-mail with infected attachments