In general internet is a 5 layer model and the model used by the ISO for making an Open systems interconnection is a 7 layer model. In ISO model, firewalls are placed in 3, 4 and 7 layers and in case of the internet model the firewalls are placed in the 3, 4 and 5 layers. These firewalls are used to protect a corporate system from the unauthorized accesses.
In simple it is said as protecting a network from the intruders and attackers. A firewall can be placed among any networks for instance a firewall can be placed between a R&D network and its corporate network related to business. Generally a firewall can be placed among the domains of lower security and higher security. The firewall system will be operating at various layers and firewall system operating particularly at layer 3 and layer 4 is sometimes called as screening router or packet filtering router. The main aim of these firewall systems is to filter the TCP ports or UDP ports and also to filter the IP packets and ICMP packets.
By depending on the filtering rules the packets are filtered as there are many ports at the router and these ports are available for routing. Packet filters can be run in the dual homed computers as they are designed in the form of software, these can filter the packets but at various networks they cannot be routed. A firewall is also sometimes called as a bastion host when it is placed in the layer 5 of Internet model and layer 7 in ISO model.
It is also called as guardian system, application gateway and proxy server. The services that are offered by the application are filtered as a main aim of these firewalls. If the firewall system is placed in the layer 2 then it is called as the link level and even it is able to operate in that link level also. For instance in order these particular assured packets are to be forwarded an Ethernet bridge is configured, which is not regular. The packets are inspected before they are transferred trough firewalls by an inspection module of checkpoint.