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A firewall is a network security device that monitors incoming and outgoing network traffic and permits or blocks data packets based on a set of security rules set inside your firewall.

Its purpose is to establish a barrier between your internal network and incoming traffic from external sources (such as the internet) in order to block malicious traffic like viruses and hackers.

The only traffic allowed on the network is defined via firewall policies – any other traffic attempting to access the network is blocked. Network firewalls sit at the front line of a network, acting as a communications liaison between internal and external devices.

Firewalls guard traffic at a computer’s entry point called ports, which is where information is exchanged with external devices. For example, “Source address is allowed to reach destination over port 22."

Think of IP addresses as houses, and port numbers as rooms within the house. Only trusted people (Source addresses) are allowed to enter the house at all—then it’s further filtered so that people within the house are only allowed to access certain rooms (ports), depending on if they're the owner, a child, or a guest. The owner is allowed to any room (any port), while children and guests are allowed into a certain set of rooms (specific ports).

A network firewall can be configured so that any data entering or exiting the network has to pass through it – it accomplishes this by examining each incoming message and rejecting those that fail to meet the defined security criteria. When properly configured, a firewall allows users to access any of the resources they need while simultaneously keeping out unwanted users, hackers, viruses, worms or other malicious programs trying to access the protected network.


Without firewalls, if a computer has a publicly addressable IP – for instance, if it is directly connected via Ethernet – then any network service that is currently running on that device may become accessible to the outside world. Any computer network that is connected to the internet is also potentially at risk for an attack. Without a firewall, these networks become vulnerable to malicious attacks.

For example:

  • If your network is connected to the internet, some types of malware find ways to divert portions of your hardware’s bandwidth for its own purposes.

  • Some types of malware are designed to gain access to your network to use sensitive information such as credit card info, bank account numbers or other proprietary data like customer information.

  • Other types of malware are designed to simply destroy data or bring networks down.

For full-spectrum security, firewalls should be placed between any network that has a connection to the internet, and businesses should establish clear computer security plans, with policies on external networks and data storage.

In the cloud era, network firewalls can do more than secure a network. They can also help ensure that you have uninterrupted network availability and robust access to cloud-hosted applications.

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