Firewall: Firewalls are software or hardware which blocks unauthorized access within or without network. Firewalls can be applied as hardware, software or sometimes both. Hardware firewalls work with embedded software (firmware) bios etc.
Firewalls are not just applications against threats from outside world. Advanced firewall systems also offer NAT (network address transformation), VPN (Virtual private networking) technologies. There are different firewall technologies to ensure security:
Application Level Firewall
Lets say a user using the local network entered the address "www.turtech.com.tr" to internet explorer and hit enter. Software is going to connect to server outside local network and will use port 80. When this request reaches to firewall. Firewall will see if it contains Http if so it will check to see if there is anything unwanted in http
Clients use a software to make this possible. This software places the information about itself in IP package. This time client adds information to applications header and firewall sees what is in ip package. (All firewalls check packages only technologies they use differ) We can use such programs to stop certain programs from using internet connection.
Packet Filtering Firewall
These types of firewalls contain some rules set by us to control up address and port number. They need to be precise therefore we need to pay extra attention. Installing such firewalls take more time. These firewalls are set up for the whole network instead of just one programme
The downside of this firewall is that you have to use extra IDS (Intrustion detection system) or IPS (Intrusion prevention system) to be able to differ normal users from attackers.