- [Instructor] A firewall is the most basic form
of defense in network security.
It provides a foundation for your overall network defense.
Therefore, it's critical to clearly understand
what a file is and learning what you can do
to improve your network security.
Before going any further,
let's define some frequently used networking terms
in this course.
Computer network refers to two or more hosts
connected through a communication medium,
like a cable or radio signal to share resources.
We leave out the word computer from now on
in our discussions of computer networks for brevity.
Hosts are any devices in the network
with a unique address.
Packets are messages containing data,
host exchange in a network.
Firewalls restrict the flow of network traffic,
by dropping suspicious attack packets
or accepting seemingly safe packets.
Firewalls also log the details of dropped packets
for a later review by administrators.
The most fundamental feature of a firewall
is filtering the packets.
There are two types of filtering going on in a firewall.
The first type is called ingress filtering,
which filters the incoming packets.
The second type of filtering is referred to as,
egress filtering, which filters the outgoing packets.
Once a packet arrives at a host,
based on its source IP address, the firewall has to decide
whether to drop the packet or forward it
to an appropriate location based on its port number.
To understand the relationship
between port numbers and IP addresses,
you need to know the different types
of addresses used in networking.
The first one is the target host IP,
which is used to reach a destination host,
for example, 10.0.0.1 is a host internet protocol
or IP address.
Please note that the IP address range we use here
is for those used only within a private network.
The second type of address is the port number.
This one is used to reach an application
after a packet arrives at a host.
For example, number 80 is used
for a hypertext transfer protocol or HTTP or web server.
Therefore based on the firewall rules,
if the port is open the packet is forwarded
to a target application on the host.
If the port is closed, the packet is dropped.
I think the relationship between a firewall and ports
it protects is a big step forward
towards mastering the topic of network security.
Congratulations on taking this crucial first step.