What is Firewall ?

1. Firewall protects internal network from the external network.

2. A firewall’s main purpose is to allow non-threatening traffic in and to keep dangerous traffic out.

3. It acts as a shield to protect your system from the untrusted, non-reliable systems connected to the internet.

4. In technical terms firewall is a piece of software hardware or both that allows only selected packet to pass from the internet to your private system.

5. A firewall is a network security device that monitors and filters incoming and outgoing network traffic based on an organization’s previously established security policies.

6. Firewall is installed on a standalone system that may or may not be networked i.e. it usually connects to an ISP.

Types of Firewall :-

1. Packet filtering firewalls

2. Application-level gateways

3. Stateful inspection firewalls

4. Circuit-level gateways

1. Packet filtering firewalls :-

• Packet filtering firewalls are the oldest, most basic type of firewalls.

• A packet filter applies a set of rules to each packet & based on the outcome, divides to either forward or discard the packet. It is also called as screening router or screening filter.

• It receives each packet & check with the rule.

• Suppose the rule is block all the packets then the firewall will block all the packets to enter the internal system.

• Packet filtering firewalls are fast, cheap and effective.

2. Application-level gateways :-

• Application gateway is also called as 'proxy server', this is because it acts like a proxy & decide about the flow of application level traffic.

• Application-level gateways protect the identity and location of your sensitive resources by preventing a direct connection between internal systems and external networks.

• Application-level gateways filter packets not only according to the service for which they are intended.

• Application-level gateways provides fine-grained security controls that can, for example, allow access to a website but restrict which pages on that site the user can open

3. Stateful inspection firewalls :-

• Stateful inspection firewalls check for legitimate connections as well as source and destination IPs to determine which data packets can pass through.

• Stateful inspection firewalls work by creating a state table with source IP, destination IP, source port and destination port once a connection is established.

• Stateful inspection firewalls create their own rules dynamically to allow expected incoming network traffic instead of relying on a hardcoded set of rules based on this information.

4. Circuit-level gateways : -

• Circuit-level gateways are quite similar to packet filtering firewalls in that they perform a single check and utilize minimal resources.

• Circuit-level gateways function at a higher layer of the Open Systems Interconnection (OSI) model.

• Circuit-level gateways are cost-efficient, simplistic and have barely any impact on a network’s performance.

• Circuit-level gateways only processes requested transactions; all other traffic is rejected.