- [Instructor] The IP routing process is very simple
and doesn't change regardless of the size of your network.
For a good example of this, let's look at the graphic
and describe step by step what happens
when Host A wants to communicate
with Host B on a different network.
First, ICMP creates an echo request payload
because this was a ping,
or what's called a packet internet groper request,
which is an echo request and an echo reply.
So Host A is sending an echo request to Host B.
So we use ICMP to create this echo request payload.
ICMP hands this payload to the IP protocol,
which then creates a packet.
The packet contains the IP source and destination address
and also has 010x in the protocol field.
Once the packet is created, IP determines
whether the destination IP address
is on the local network or a remote network.
Since it is a remote request,
the packet must be sent to the default gateway,
so we're gonna look in the registry
to find the default gateway IP address.
Then Host A is going to ARP for it,
or it's gonna look in its ARP cache first
and then ARP for the MAC address
of ethernet zero on router B at this point.
Once the packet and destination hardware address
are handed to the data link layer,
the LAN driver is used to provide
media access via the type of LAN being used,
which is ethernet in this case.
Once the frame is completed,
it's handed down to the physical layer
to be put on the physical medium.
And since we're using an ethernet with twisted pair,
we're gonna send it down using Manchester encoding
one bit at a time.
Every device in the collision domain
receives these bits and builds the frame.
They each run a CRC and check the answer
in the frame check sequence field.
The router's ethernet interface
is going to receive the bits,
put them into bytes and build the frame and run a CRC.
If the hardware address matches,
then the ethertype field is checked
to find the protocol used at the network layer,
which in this case is IP.
The packet is pulled from the frame,
and the frame is discarded.
It's important to remember here guys
that the frame was used to take the packet from the host
and hand it to the router,
and then the packet was discarded, not to be used again.
We're going to route this packet now
to the exit interface of e1 heading towards Host B,
and we're going to build a new frame.
The source MAC address will be e0,
the destination MAC address will be Host B.
Host B is going to pull the packet from the frame
and discard it.
Frames are one use.
We don't have a very green network here, my friends.
We discard these encapsulations, right?
They're one use.
It's important to remember on this
that frames and MAC addresses are only local
and that IP addresses go from end to end
so we can find the destination host.
Now Host B is going to make an ICMP echo reply,
put it in a frame with a source MAC address of Host B
and a destination MAC address of e1 in this example.
The packet will be sent to router B,
the packet will be pulled from the frame
and the frame discarded.
We're going to packet switch it to e0.
Then we're going to go ahead and frame it again
with a source MAC address of e0
and a destination MAC address of Host A.
The key point here is that if you had a much larger network,
the process would be the same.
It's just that the larger the inner network,
the more hops the packet goes through
before it finds the destination host.
It's super important to remember
that when Host A sends a packet to Host B,
the destination hardware address used
is the default gateway's ethernet interface.
You ask why?
Because only local networks care about MAC addresses.
So packets destined for remote networks
must go through the default gateway.