How can Firewall Protect your computer ?

The first thing necessary is to understand what a firewall is,and what it's function is.The whole point of a firewall is to completely isolate your computer from the internet.Everything that passes in or out of your computer hits this wall to see if it can pass or needs to be blocked.The information that passes through a computer is made up of packets.Each packet passes from one machine to another in a network.So the connections,computer savvy people talk about are merely the passing of packets of information to and from other computers.This is used to send or receive a packet of information no matter if one computer is two feet or another country from destination computer.Every internet packet has to have an adrdress and port to go.It must also have the port and IP address that it is being sent from to be recognized by the other computer.It is the same as having a letter with the address it is going to and the address of who sent it.
Firewall software inspects these packets of information as they try to pass through.As the data arrives at your computer the firewall looks at it before any other program on your computer.If the packet looks suspicious it can deny entry to the rest of the computer so infection can be avoided.A port is only open when the first arriving packet tries to connect.If it is ignored or denied the port is now closed and nothing else can get into it.It is great tool to have because it is very selective about what it will allow in and what it blocks.It can recognize IP addresses and know which ones are safe and which ones are not.What this means is when you are running on a specific port all information can only come through that port and no others.Even if you had a trojan that opened a listening port it would not be detected due to the fact that its port could not be accessed.The firewall would block the signal.