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- [Instructor] A firewall is the most basic form

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of defense in network security.

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It provides a foundation for your overall network defense.

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Therefore, it's critical to clearly understand

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what a file is and learning what you can do

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to improve your network security.

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Before going any further,

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let's define some frequently used networking terms

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in this course.

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Computer network refers to two or more hosts

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connected through a communication medium,

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like a cable or radio signal to share resources.

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We leave out the word computer from now on

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in our discussions of computer networks for brevity.

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Hosts are any devices in the network

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with a unique address.

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Packets are messages containing data,

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host exchange in a network.

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Firewalls restrict the flow of network traffic,

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by dropping suspicious attack packets

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or accepting seemingly safe packets.

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Firewalls also log the details of dropped packets

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for a later review by administrators.

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The most fundamental feature of a firewall

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is filtering the packets.

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There are two types of filtering going on in a firewall.

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The first type is called ingress filtering,

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which filters the incoming packets.

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The second type of filtering is referred to as,

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egress filtering, which filters the outgoing packets.

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Once a packet arrives at a host,

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based on its source IP address, the firewall has to decide

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whether to drop the packet or forward it

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to an appropriate location based on its port number.

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To understand the relationship

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between port numbers and IP addresses,

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you need to know the different types

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of addresses used in networking.

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The first one is the target host IP,

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which is used to reach a destination host,

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for example, 10.0.0.1 is a host internet protocol

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or IP address.

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Please note that the IP address range we use here

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is for those used only within a private network.

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The second type of address is the port number.

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This one is used to reach an application

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after a packet arrives at a host.

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For example, number 80 is used

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for a hypertext transfer protocol or HTTP or web server.

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Therefore based on the firewall rules,

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if the port is open the packet is forwarded

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to a target application on the host.

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If the port is closed, the packet is dropped.

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I think the relationship between a firewall and ports

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it protects is a big step forward

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towards mastering the topic of network security.

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Congratulations on taking this crucial first step.