the next topic of discussion
is going to be the process of encapsulation and de encapsulation
art or the our process R stands for
address resolution protocol. Actually, before I move on to their, I should mention that there is a site online where you can go practice your sub netting
and the site is called some netting questions dot com. Great site. You go on that. It asked you a question, and then you say show answer. I'll show you the answer and then you do the next question and so on and so forth.
So the site is submitting questions dot com. So before we move on to the end, Cap de encapsulation and our process,
I wanted to show you what the protocol data unit called a packet at Layer three looks like,
pretty much looks like a like a frame except the address fields.
Instead of a Mac address, you have an I P address. And again the first field is the destination I p address field. The second field is the source I P address field.
Then you have the data field and the frame check sequence. The frame check sequence houses the cyclic redundancy check, and it works exactly the same way as it does with the frame onto encapsulation de encapsulation and AARP Host one and host too
are residing on the same land. Sometimes I get asked, the question is host one and host you. Are they connected to a switch? Is there a switch in the middle? The answer is most definitely Yes.
But however, here I have drawn a logical dry graham, not a physical diagram. In a logical diagram, you exclude switches.
So yes, there is a layer to switch here in the middle. But I have excluded that from this diagram Host ones I p addresses 10 10 10 1 and host, whose I P address is 10 10 10 to host. One wants to ping host too.
I have to talk about what Ping is.
Ping is basically a testing utility, and Ping works with the protocol
Internet Control Messaging Protocol, or ICMP. I see him. Ping has two parts and echo and an echo reply. So basically, if Host One wants to ping host to on host one from the command prompt you were typing
And Host one would send out an Echo ICMP message down to host two and host to Will read that message and send back an echo reply or icmp Echo reply. Having said that, let's get back to the diagram Host one wants to playing host to
So does host one. No, its own I p address. Yes, just host one knew its host, Whose I p address. Yes. So Host one creates a packet with the source i p 10 10 10 1 and Destination I p 10 10 10 to
now let's assume host ones. Once again, Mac address is a And of course, as always, this is not 48 bits. It's only 16 bits, but we're going to assume it's a
we're gonna pretend this is 48 bits
and host Who's Mac address is B B B B Host one has already created a packet with source I P 10 10 10 1 and Destination I P 10 10 10 to
next to transmit over the local data link. Remember, even though there's a switch in the middle boat Host one and host who are on one data link,
a switch is one data link.
By default, Host one needs to create an Internet frame
because you use their two for transmission or the local data link. So Host one needs to create and eaten it frame.
Do. Send that packet over to host, too.
Host One needs host. Who's Mac address to create the friend host. One already knows its own Mac address is a butt host. One does not know host who's Mac address, which is B B B B.
So at this point, Host One generates an address resolution protocol request,
which is a broadcast.
So Host one generates
requesting for the Mac address off the device with I p
10 to. So host one is actually saying, whoever has I p 10 10 10 to please send me your Mac address.
Now this our request is a broadcast. It is actually sent to the destination address. 255
255 Which is the all networks, all sub nets broadcast address. Everybody's gonna hear it. If there were other hosts here, they would hear it too. But I'm trying to keep it simple.
Do you explain this concept. Host one generates an art request, which is a broadcast asking for the Mac address of the device with the I P 10 10 10 to
and host one transmits that broadcast host who receives that broadcast
its own I p is 10 10 10 to
so Host two replies through the Our broadcast, and the R
reply is actually a unique cast back to the I P address of Host One, So the odd reply will be sent to 10 10 10 1
Host two replies with its own Mac address in the art reply, which is B B B B.
Host one has the Mac address off host to host. One has its own Mac address and host one also has the Mac address off host, too.
This is the function of our are basically resolved. I p addresses to their relevance Mac addresses and then stores it in a table so this process will only happen once on Host one. Once host one has host two's Mac address, resolved once it has a binding in the table that say's
I P 10 10 10 to has a Mac
It won't need to go through this. Our process a second time.
Next host one in capsule eights,
the original ICMP packet and puts it inside of the Internet frame that it created the Internet frame that it created as a sore smack off A and destination Mac of B B B B.
So the frame would look something like this.
So the destination Mac would be be be be be
the source. Mac would be a a
the frame check sequence housing the CRC. As I said, Host one will encapsulate the original ICMP packet inside of the frame and put it inside the data portion of the frame
so the package will actually be sitting inside off the frame. The destination I p would be 10 10
to the source. I p would be 10 10
And then you have the data and the frame check sequence housing the C. R. C. The act of encapsulation means I take the layer three packet and I put it in the data portion of the frame,
which is what we see here. This is the data portion of the frame. Host one will then transmit the frame over to host to host. Two will receive the frame and match the destination. Mac address field in the frame with its own Mac
once it matches Host Who will discover or know that this frame belongs to me. It was meant for me
host to At this moment will de encapsulate the frame and extract the original packet. Basically discard the frame and extract the original packet.
Then host to will match the destination i p address
in the packet with its own I P address, which is docked to hair and not to hear since it matches host to will send back the ICMP echo reply
the AARP encapsulation the encapsulation portion of this lecture.
Next we will learn how to configure I P addresses on our routers and then we will see this process in action