1) Firewall (as iptables): A Firewall is quite simply a TCP/IP host on the internet, with multiple IP interfaces or network cards. One interface is connected to the internet, and other is connected to the internal network.
Firewalls are intended to keep the flames of internet hell out of your private LAN, or, to keep the members of your LAN pure and chaste, by denying those accesses to all the evil internet temptations.
Iptables now uses different tables for different action. Generally filter and nat tables are used. There are five built-in chains (Input, Output, Forward, Prerouting, and Postrouting). The chains represent special hooks in the netfilter code that are used for the packet filtering.
A firewall is a single security point of a network that filters unwanted packets and unauthorized users. In Linux, we use IP Table software as a firewall.
The IP Table is a command line firewall
The IP Table also uses a network layer and is known as the Packet filtering Firewall.
Each packet has following information into their headers field:
Five services addressing security mechanisms for client-server functions are discussed.