This lesson explains the fundamental steps a host takes to communicate with another host on the same network, focusing on the processes at Layer 3 (IP addressing) and Layer 2 (MAC addressing), particularly the role of ARP.
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hello welcome to another lesson for my
new course on networking fundamentals
the first module of this course will
teach you everything you need to
understand how data flows through the internet
internet
in this lesson we'll be taking a look at
the internet from the host's perspective
we're going to look at everything a host
does to speak on the internet
before watching this video it is
recommended to have watched the prior
lessons in this module
in lesson one we unpacked the concepts
of a host ip address
network switches and routers and in
lesson two we took a practical look at
the osi model
and showed you how each layer
contributes to the overall goal of
networking which is moving data between hosts
hosts
there will be links to each of these
lessons in the description in this
lesson we're going to illustrate
everything hosts do to speak on the
internet and we're going to do this
across two scenarios
the first scenario will outline
everything that occurs for two hosts
which are directly connected to one
another to communicate
in the second scenario we'll outline
everything that occurs for two hosts to
communicate on opposite sides of a router
router
now regarding this first scenario hosts
don't actually know what they are
connected to
so whether host a is directly connected
to host b or whether there is one switch
or multiple switches in between
either way host a is going to follow the
same steps so this first scenario is
actually showing you
everything that hosts do when
communicating with other hosts on the
same network
regardless of how they are connected and
regarding that second scenario
hosts also don't know whether what they
are trying to speak to is on the other
side of one router
or multiple routers or on opposite sides
of the internet
so again we'll remove all the extra
variables to keep it simple
which will tell us exactly what a host
does to speak to any other host on a
foreign network
these two scenarios will illustrate 100
of the communication that occurs
on the internet from the perspective of
each host and we're going to be doing
this in two parts
in part one in this video we'll show you
all the steps hosts take to communicate
to other hosts on the same network
and in part two in the next video we'll
show you how it changes if you add a
router to the mix
that being said let's jump right into it
with our first illustration
so here we have two hosts and these two
hosts are directly connected to each other
other
now they don't know that so what they're
going to be doing is identical if there
were hubs or switches in the mix
both these hosts have a nic which this
wire is physically connected to
which means both these hosts have a mac
address i'm providing
only the first four digits of each
host's mac address to conserve screen
real estate
moreover both these hosts have ip
addresses and subnet masks
we defined earlier in this module that
ip addresses serves as the identity of
each host
we didn't yet give you a definition for
a subnet mask but a subnet mask
essentially tells you the size of a
particular network
this is all done through a process known
as subnetting and we're not going to be
covering subnetting in this module
if you're interested in learning
subjecting go and check out the free
videos we've created at subnet ipv4.com
host a has some data it needs to send to
host b
now from the network perspective we
don't know or care what that data is
it's just a bunch of ones and zeros that
needs to go from host a
to host b host a also knows the ip
address that it's trying to speak to
it knows the ip address of host b now
how it knows that is somewhat outside
the scope of this video
but the simplest explanation is maybe
the user of host a
jumped on the command prompt and typed
ping 101.33
this provided the ip address to host a
that needs to receive this data
alternatively maybe host a acquired the
ipedar suppose b
through dns dns is a protocol which
converts a domain name into an ip address
address
for example when you jump on your web
browser and type in practicalnetworking.net
practicalnetworking.net
dns is what's going to turn this domain
name into an ip address that your
computer can send a web request to
one way or another host a is going to
use something like this
in order to know the ip address that
it's trying to send the data to
moreover host a also knows that the ip
address that it's trying to speak to
is on host a's own ip network host a
will do this by looking at its own ip
address and comparing it to its own
subnet mask
to determine how many other ip addresses
are on the same network
now all this again is done through
subnetting which we won't be covering in
this module
for now just take my word for it that
host a knows that 10-133
is on the same ip network of host a if
you want to learn more about subnetting however
however
feel free to check out the free videos
at subnet ipv4.com
either way since host a knows the ip
address who it's trying to speak to
it's able to create the layer 3 header
to attach
to this data this layer 3 header is
going to serve the function of end to
end delivery
therefore it's going to include a source
ip address of whoever is sending the data
data
in this case host a and a destination ip address
address
of whoever is meant to receive the data
now of course
layer 3 cannot interact with a wire for
that we need layer 2.
so host a next needs to add a layer 2
header to this packet
the problem is that host a does not know
host b's mac address if we look back at
our example of how host a
new host b's ip address many of you have
used the ping command
and i imagine all of you have typed in
ping and then an ip address
but notice you never typed in ping an ip
address and a mac address you only provided
provided
the ip address to the computer the
computer figured out the mac address on
its own
and that's exactly what host a is going
to have to do host a is going to have to
use the address resolution protocol
or arp to resolve host b's mac address
this in fact is exactly the purpose of
arp arp is meant to link a layer 3
address to a particular layer 2 address
so let me show you how that works
host a needs to use arp to resolve the
target ip addresses mac address
meaning that mac address right there
what it's going to do
is shoot out an arp request this arp request
request
is going to ask for the mac address
which is associated with a particular
target ip address
in this case the ip address we are
trying to resolve 101.33
moreover this arp request is going to
include the
sender's mac address host a is going to
include its own ip address
and mac address in the request itself
this will be useful so that host b
can respond directly to host a later on
now notice that the arp request included
a layer 2 header
this layer 2 header is meant to take the arp
arp
payload and get it delivered to host b
but that layer 2 header
does not have a destination mac address
of b's mac address
if host a new host b's mac address it
wouldn't have to do arp to begin with
and could just populate the mac address
in this layer 2 header from the very
beginning instead
this layer 2 header is sent as a
broadcast meaning
it's sent to everybody on the network
what that means is that it has a
destination mac address of all
fs the all f's mac address is a
specially reserved mac address which is
meant to be used
whenever you want to deliver a packet to
everybody on the local network
that is by definition a broadcast now if
you're interested in learning more about broadcast
broadcast
i created another video which discusses
local broadcast and directed broadcast
there'll be a link for that video in the
description as for us we're going to continue
continue
arp mappings are stored in what's called
an arp
cache every device which has an ip
address has an
arp cache host a has an ip address and
host b has an ip address therefore they
both have
arp caches at the moment host a's arp
cache simply states that we're trying to resolve
resolve
the 10 1 1.33 ip address to a particular
mac address
host b's arp cache is currently empty
when this request
however makes it across the wire you'll
see something happens to host b's arp cache
cache
notice host b was able to populate an entry
entry
in its arp cache for host a it was able
to identify
that the ip address 101.22 maps to the
mac address
a2 a2 that occurred because
in the original arp request host a
provided its own
mac address so by simply receiving an
arp request
host b was able to learn an arp entry
host b
will now respond to that arp request by
sending what's known as an arp response
that arp response will include the
mapping that host a was trying to resolve
resolve
specifically the ip address 10133 and
the mac address that owns it
this arp response is sent unicast meaning
meaning
directly back to host a since host b
knows host a's mac address
it can create the layer 2 header which
will take the arp response
directly to host a and not have to
interrupt everybody on the wire by using
a broadcast
either way when that arp response
arrives on host a
host a is able to create the arp mapping
that was listed in the arp response
now host a has everything it needs to complete
complete
the layer 2 header for the data it was
initially trying to send to host b
the layer 2 header is going to include a
source mac address of host a's mac address
address
and a destination mac address of host
b's mac address
this layer 2 header will accomplish the
goal of hop
to hop delivery which finally
lets this data be sent to host b
upon arriving to host b host b will
discard the layer 2 header
again the whole purpose of that header
was to get the packet from this nick
to this nick it did so successfully and
can now happily retire in header heaven
next host b will discard the layer 3
header again the only purpose of that header
header
was to get this data from host a to host b
b
and it did that successfully and can
therefore also retire in header heaven
and finally the application on host b
can process
the data that it just received so that
is what is required
to get some data from host a to host b
now presumably host b is going to have a response
response
meaning it's going to have some ones and
zeros it's going to have to send
to host a but the good news is host b
has all the necessary information
in order to respond much quicker than it
initially took
host b knows who it's trying to
communicate so it's able to create the
layer 3 header
with a source ip address of host b and a
destination ip address post a
and since host b's arp cache is already populated
populated
it's able to create a layer 2 header
that includes
its own mac address as the source and
host a's mac address
as the destination which it knows
because it's stored in its arp
cache in fact any further data exchange
between these two hosts
now happens very easily because both
hosts have
everything they need in order to create
the layer 3
and layer 2 headers successfully and that
that
wraps up our lesson on everything hosts
do to speak to another host on the same
network the key takeaways of this lesson
is understanding all the steps a host
takes when speaking to another host on
the same network
understanding that those steps are the
same regardless on whether there are
switches or hubs in between the two hosts
hosts
and finally understanding arp and how it
works and how it links
a layer 3 address to a layer 2 address
in the next video we're going to add a
router between these two hosts
to show you how these steps change this
will illustrate everything a host does
to speak to another host on a foreign network
network
but that wraps up this video i hope you
enjoyed this video
i want to thank you for watching and
i'll see you in the next video as we unpack
unpack
how hosts communicate through a router
hey youtube i hope you enjoyed that free
lesson for my new course on networking fundamentals
fundamentals
i'll be releasing the entire first
module for free here on youtube
i want this course to be the ultimate
networking fundamentals course and since
i'm still scoping out the outline you
could have a say in what topics will be covered
covered
let me know in the comments below what
subjects you want included in this course
course
otherwise remember to like and subscribe
and of course if you learned something
from this video
the best way to thank me is to share
this video it's a small act of gratitude
but one i appreciate greatly
i hope you enjoyed this lesson i want to
thank you for watching and we'll see you
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