FIREWALLS!

What is a firewall

First of all it is important to understand in basic terms what a firewall is. In agricultural terms a firewall is a piece of land, normally a pretty wide dirt road, that would not allow a fire in one field to move and set on fire the next field, due to the fact that a dirt road has no parts (like leaves or dry wood) that can "carry" the fire.
In computer terms a firewall is a system (either hardware based or software based) that divide, actually isolate the two fields, one being the external world ie. the Internet, and the other, the internal world or computer or computer network.

Opening a number of ports in the firewall, we can allow information to go from the internet to our network (inbound traffic) or from our computer or network outside (outbound traffic).
 

Firewalls use one or more of three methods to control traffic flowing in and out of the network:

• Packet filtering - Packets (small chunks of data) are analyzed against a set of filters. Packets that make it through the filters are sent to the requesting system and all others are discarded.
• Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa.
• Stateful inspection - A newer method that doesn't examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information traveling from inside the firewall to the outside is monitored for specific defining characteristics, then incoming information is compared to these characteristics. If the comparison yields a reasonable match, the information is allowed through. Otherwise it is discarded.

Controlling the packets of data that goes thru a firewall, allows to block potential hackers to infiltrate your network and also to control in both directions the use of the internet connection.