Firewall is a system that is present between external network i.e. Internet and internal network LAN. It allows your internal network to access the internet but it restrict external network so that not one access you LAN systems or data on your internal network. Firewall can also be defined as a device configured to allow and reject traffic on the basis of defined rules.
How it Works
Firewall stops all incoming traffic to internal IP addresses. Let us take an example of ping to understand the working of firewall. Whenever a ping command is executed it sends in ICMP echo request messages to source and in return receives ICMP echo reply. To block these ICMP echo request messages to reach destination we will configure firewall to fail the ping commands. Similarly SMTP protocol that response to TCP port 25 is used for email, blocking the port will disable the user in network to send emails.
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