A firewall is a part of a computer system used to prevent any unauthorized access and allows safe communications. It can be implemented by hardware or software, or combination of both.Each firewall has different kind of authorisation criteria.

The firewall techniques consists of four main types namely

1. Packet filter

2. Application gateway

3.Stateful firewall

4. Proxy server

Packet filter

The data transfer between computers and networks takes place by a basic unit called packets. Packet filter uses a set of user defined rules for identifying the source address, destination address and port number.If a packet matches the rules, the filter either discards it or rejects it. Packet filters are not tough to configure. They are also susceptible to IP spoofing.

Application gateway

They are located between end user and network. The end user contacts the gateway ,after which it performs the required function for the user.Specific programs use specific mechanisms. This type is simple,since it only functions to proxy requests from end users. It also intercepts IP packets from the net.

Stateful firewall

It is used to keep track of the network connections moving across it. It distinguishes legitimate packets for different connections and only allows packets matching a specific connection state. All others will be rejected.This type of firewall is advantageous as opposed to packet filters since it need only check the connection against its table rather than an extensive rule set.

Proxy server

Proxy server is one of the most favourable types, they act as go-betweens for outside clients seeking information from servers.The request is filed and checked according to various filtering rules hence filtering traffic from certain IP addresses. The  direct access to the server is subsequently handled by the proxy on the behalf of the clientThe servers are hence kept safe and anontmous from any attack.