We've taken a look at the seven layers of the Open
Systems Interconnection model, but as we said,
we don't really use it, except as a reference.
Instead, for the most part, we use TCP/IP.
TCP/IP actually was developed before OSI,
but some of the problems with TCP/IP are that we can't even
agree on what the various layers should be called or even
sometimes how many layers there are.
Many people argue there should be three, four, or five layers to TCP/IP.
It doesn't matter.
The whole point is that's why we often use OSI because
it is more defined and structured.
We used the four‑layer model here because that came out of the
original development of the DARPA and DARPANET projects,
but that included then taking the top three layers of the OSI model,
application, presentation, and session,
and combining them into the application layer and taking the bottom two layers,
data link and physical, and combining them into the network access layer.
One of the things that makes the internet work for you
and me is the Domain Name System.
You see, you and I like to talk in regular,
in my case, English, but the internet, routers for example,
are based on binary values, they don't understand English.
So, we use a natural name that needs to be then translated into a binary value,
and that is done by DNS.
DNS takes that natural name that we type in,
pluralsight.com,
and converts it into the IP address of pluralsight.com so routers know how
to route that traffic towards its correct destination.
For example, if you typed in www.apple.com,
it would be then translated into the IP address of 17.253.144.10.
The problem, of course, is that if DNS goes down,
then really, most users won't be able to get to a website.
If you knew the IP address, 17.253 for example,
you could get to Apple without ever typing in apple.com.
But since very few of us know what those IP addresses are,
we rely on DNS as really a telephone book that has conversion that says,
this is the number that belongs to this name.
One of the things that makes the internet work is the use of ports.
Ports, we could call them doorways.
They're used to control traffic and there's a number of
common ports we use all the time.
For example, if I'm going to transfer files,
I'll very often use a protocol known as File Transfer Protocol
that runs on the doorway marked 20 and 21.
If I'm going to transmit traffic using a secure shell,
that would be port number or door number 22.
TELNET, terminal emulation network, 23; email,
Simple Mail Transfer Protocol, runs on port 25.
The DNS, which we just looked at, requires port 53 to be open,
and that's where DNS traffic and resolvers would come in.
So if I typed in apple.com,
the DNS system could communicate over that door to be
able to tell us what the IP address was.
And the most common one we use when we're surfing the internet,
of course, is port 80, which is used for hypertext transfer protocol traffic.
We also have Post Office Protocol 3, which runs on port number 110,
and when we're sending encrypted traffic,
say to a bank, we use Transport Layer Security,
TLS, which was an adaptation and improvement on the former SSL,
Secure Sockets Layer, which runs on port 443.
Where are these doors located?
On a firewall.
The firewall has a number of doors that are open to certain traffic,
but will block traffic to destinations and types of traffic that it says,
no, I'm not going to allow any traffic through a different port.
The Key Points Review.
We've covered a tremendous amount of network terminology
and theory here in just a few minutes,
but it's good to go over this again and make sure I've understood
it if I'm not really familiar with networks,
because I can't really secure a network until I have some
understanding of how a network works.
We saw how OSI is an excellent reference model that will help us
understand the process of network‑based communications.
TCP/IP is the basis used by many devices today and systems to communicate,
even including things like our cars, for example,
many of them run almost entirely on TCP/IP communications today.