Firewall

What is a firewall?

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. 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.

A firewall is a security system that monitors and controls network traffic based on a set of security rules. Firewalls usually sit between a trusted network and an untrusted network; oftentimes the untrusted network is the Internet. For example, office networks often use a firewall to protect their network from online threats.

Firewalls decide whether to allow incoming and outgoing traffic to pass through. They can be built into hardware, software, or a combination of both.

Firewalls carefully analyze incoming traffic based on pre-established rules and filter traffic coming from unsecured or suspicious sources to prevent attacks. Firewalls guard traffic at a computer’s entry point, called ports, which is where information is exchanged with external devices. For example, “Source address 172.18.1.1 is allowed to reach destination 172.18.2.1 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 (destination address) at all—then it’s further filtered so that people within the house are only allowed to access certain rooms (destination 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).

Packet-filtering firewalls, the most common type of firewall, examine packets and prohibit them from passing through if they don’t match an established security rule set. This type of firewall checks the packet’s source and destination IP addresses. If packets match those of an “allowed” rule on the firewall, then it is trusted to enter the network.

Here are eight types of firewalls:

• Packet-filtering firewalls
• Circuit-level gateways
• Stateful inspection firewalls
• Application-level gateways (a.k.a. proxy firewalls)
• Next-gen firewalls
• Software firewalls
• Hardware firewalls
• Cloud firewalls
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Understanding Packet-Filtering Firewalls

Packet-filtering firewalls validate packets based on protocol, source and/or destination IP addresses, source and/or destination port numbers, time range, Differentiate Services Code Point (DSCP), type of service (ToS), and various other parameters within the IP header.

The primary advantage of packet-filtering firewalls is that they are located in just about every device on the network. Routers, switches, wireless access points, Virtual Private Network (VPN) concentrators, and so on may all have the capability of being a packet-filtering firewall.