- [Teacher] In the DoD model, there are two main reasons
for the internet layer's existence: routing, and providing
a single network interface to the upper layers.
None of the upper or lower layer protocols
have any functions relating to routing.
That complex and important task belongs
entirely to the internet layer.
Without this layer, application programs would need
to write hooks into every one of their applications
for each different Network Access protocol.
This would not only be a pain in the neck,
but it would lead to different versions of each application,
one for ethernet, one for wireless, and so on.
To prevent this, IP provides one single network
interface for the upper layer protocols.
All network roads don't lead to Rome, they lead to IP.
And all the other protocols at this layer
as well as all those at the upper layers use it.
Never forget that.
All paths to the DoD model go through IP.
Some of the protocols of the IP are Internet Protocol,
ICMP, or Internet Control Message Protocol,
and Address Resolution Protocol.
IP, or Internet Protocol, operates
at the internet layer of the TCP stack.
It is connectionless.
Now it's important to remember that the only
connection-oriented protocol in the DoD stack is TCP.
So any other protocol is connectionless, including IP.
IPv4 is a hierarchical addressing scheme.
We have best-effort delivery with IP.
No data recovery features because it's connectionless.
Physical media independent, doesn't matter what it runs on.
And the two routed protocols we use today are IPv4 and IPv6.
The Internet Protocol essentially is the internet layer.
The other protocols found here mainly exist to support it.
IP holds the big picture and can be said to see all
because it's aware of all the interconnected networks.
It can do this because all the machines on the network
have a software or logical address called an IP address,
which we'll explore more later.
For now, understand that IP looks at each packet's address,
then using a routing table, it decides where the packet
is to be sent next, choosing the best path to send it upon.
Let's take a look at an IP header.
In the IP header, it's not important
that you remember every field here.
But let's go through a couple that you should know.
First off, we have our IP address, source, and destination.
Each of these are 32 bits long.
We also have something that I want you to understand,
it is called the protocol field.
This defines the data, so if my data is here,
we're looking at the header, and it has data in tow,
the protocol field says, "Who owns this data?
"Who did I get it from?"
Well, it could be something simple like TDP or UDP,
or it could've been ICMP data, for example.
The last field I wanna talk about here is the time to live
field, also just referred to as the time to live.
This says, "How far can a packet go,
"meaning how many routers can I go through?
"How many hops can I go through before the packet dies?"
That becomes important in our diagnostics.
Now let's go back to the protocol field
and talk about what we can find in there.
Looking at this other graphic, we can see that if it's
TCP data, then we're gonna have a hex of 0x06.
That defines that we're carrying a TCP segment.
If it was UDP, we'd have 0x17, defining we have UDP segment.
If we have ICMP, for example, it would be 0x01.
Now that we've taken a look
at the different layers of the DoD stack, understand
there's no protocols defining the network access layer.
So let's jump right into IP addressing.