Everything Hosts do to speak on the Internet - Part 2 - Networking Fundamentals - Lesson 3
Summary
TLDRThis lesson from the networking fundamentals course delves into how hosts send data on the internet. It builds upon previous concepts, focusing on host communication within the same network and with foreign networks. The video explains the importance of ARP cache, demonstrating how hosts use it to resolve MAC addresses for local and foreign network communication. It also clarifies the role of subnetting in determining network boundaries and the necessity of knowing a router's IP address as the default gateway. The lesson is crucial for understanding how data moves across networks and the foundational steps hosts take to communicate effectively.
Takeaways
- đ The course module covers how data flows through the internet, focusing on hosts and their communication.
- đ It's essential to watch previous lessons for foundational concepts before diving into this video.
- đĄ Hosts use IP and MAC addresses to identify and communicate with each other on the network.
- đ The subnet mask defines the size of a network, and subnetting is crucial for understanding how hosts determine the network they belong to.
- đš Hosts use ARP (Address Resolution Protocol) to resolve the MAC address of the next hop, which is necessary for sending data.
- đ When communicating with a foreign network, hosts use their default gateway's IP address, which is configured as the router's IP address.
- đŹ ARP cache is vital for hosts to keep track of IP-to-MAC address mappings for efficient communication.
- đ Once the ARP cache is populated, hosts can reuse the MAC address for the default gateway to communicate with any host on foreign networks.
- đ The lesson emphasizes the importance of understanding how layer 3 (IP) and layer 2 (MAC) headers are used in data transmission.
- đ The next lesson will explore the roles of switches and routers in facilitating network communication.
Q & A
What is the main focus of the first module in the networking fundamentals course?
-The first module of the networking fundamentals course focuses on teaching how data flows through the internet.
What is recommended before watching this particular video lesson?
-It is recommended to have watched lesson 1 and lesson 2 from the series, as this lesson elaborates on many concepts introduced in prior lessons.
What is the purpose of the subnet mask mentioned in the script?
-The subnet mask defines how big a particular network is, and it is a function of subnetting.
What is an ARP cache and why is it important?
-An ARP cache is a table that stores the IP address to MAC address mappings for network devices. It is important because it allows a host to communicate with other devices on the network by resolving their MAC addresses.
How does a host know if it needs to communicate with a device on a foreign network?
-A host determines if it needs to communicate with a device on a foreign network by comparing its own IP address and subnet mask with the target IP address.
What is the role of a default gateway in a host's network communication?
-The default gateway is the IP address of the router that a host uses to communicate with devices on foreign networks. It is configured on the host and used to resolve the router's MAC address via ARP.
Why does a host need to create a layer 3 header when sending data to another host?
-A host needs to create a layer 3 header to identify the source and destination IP addresses for the communication, which is necessary for routing the data to the correct destination.
What is the purpose of the layer 2 header in the context of the video?
-The layer 2 header is used to facilitate the hop-to-hop delivery of packets within a network, directing the data to the next device in the path, such as a router.
How does a router handle the packet once it receives it from a host?
-Upon receiving a packet, a router discards the layer 2 header, which was used to get the packet to the router, and then it takes over to route the packet to the next hop, which could be another router or the final destination.
What is the significance of the ARP entry for the default gateway?
-The ARP entry for the default gateway is significant because once resolved, it can be reused for all communications with hosts on foreign networks, simplifying the process of sending data off the local network.
What will be the focus of the next lesson according to the script?
-The next lesson will focus on everything switches and routers do to facilitate communication within and between networks.
Outlines
đ Understanding Host Communication on the Internet
This paragraph introduces the topic of how hosts communicate over the internet, focusing on the process of sending data from one host to another on different networks. It emphasizes the importance of understanding the basics of networking fundamentals, particularly the role of hosts in data transmission. The lesson builds upon previous lessons and introduces the concept of routers and subnetting. The video explains that hosts use both MAC and IP addresses to communicate and that the ARP cache is essential for resolving IP addresses to MAC addresses. It also touches on the concept of subnet masks and how they define network sizes, noting that subnetting will not be covered in this module. The lesson concludes with a demonstration of how Host A communicates with Host C on a foreign network, illustrating the process of creating layer 3 and layer 2 headers and the use of ARP to resolve the router's MAC address.
đ The Role of ARP in Network Communication
This paragraph delves into the role of the Address Resolution Protocol (ARP) in facilitating communication between hosts on different networks. It explains how ARP is used to resolve the MAC address of the default gateway when a host needs to communicate with a host on a foreign network. The paragraph also discusses the process of creating layer 2 headers for data packets, which are necessary for hop-to-hop delivery. The video script illustrates how once a host has resolved the MAC address of the router, it can reuse this information to communicate with any host on foreign networks, highlighting the efficiency of the ARP process. The lesson wraps up by emphasizing the importance of understanding how hosts communicate with other hosts on both local and foreign networks, and how ARP plays a crucial role in this process. The next lesson is teased, focusing on the functions of switches and routers in facilitating network communication.
Mindmap
Keywords
đĄNetworking Fundamentals
đĄData Flow
đĄHosts
đĄInternet Protocol (IP) Address
đĄMAC Address
đĄSubnet Mask
đĄARP Cache
đĄLayer 3 Header
đĄLayer 2 Header
đĄRouter
đĄARP Request
đĄDefault Gateway
Highlights
Introduction to the course on networking fundamentals focusing on how data flows through the internet.
Lesson prerequisites include watching lesson 1 and lesson 2 for foundational concepts.
Illustration of how hosts communicate within the same network, emphasizing the role of MAC and IP addresses.
Explanation of subnet masks and their function in defining network size, with a note on subnetting.
ARP cache's importance in identifying IP addresses with MAC addresses for communication.
Host A's process of sending data to Host C on a foreign network, involving creating a Layer 3 header.
Understanding how Host A determines if Host C's IP address is on a foreign network through subnetting.
The necessity for Host A to create a Layer 2 header for hop-to-hop delivery to the router.
Host A's use of ARP to resolve the MAC address of the router when the ARP cache is empty.
The configuration of a default gateway on Host A and its significance in network communication.
Demonstration of how Host A populates its ARP cache with the router's MAC address.
The role of the router in discarding the Layer 2 header and facilitating further data transmission.
Highlighting the reusability of the ARP entry for communication with any host on foreign networks.
Explanation of how the Layer 2 header remains the same for communication with any host on a foreign network.
The critical role of ARP in determining if the target IP is on the local or foreign network.
Summary of the lesson's key points: understanding host communication on local and foreign networks and the role of ARP.
Anticipation of the next lesson focusing on the functions of switches and routers in network communication.
Invitation for viewers to participate in shaping the course content by commenting on desired topics.
Encouragement for viewers to like, subscribe, and share the video as a form of appreciation.
Transcripts
hello welcome to another lesson from my
course on networking fundamentals
the first module of this course will
teach you everything you need to
understand how data flows through the
internet
in this video we're going to continue
our lesson discussing everything hosts
do
to send data on the internet and before
watching this video
it is recommended to have watched lesson
1 and lesson 2 from the series
this lesson uses and elaborates on many
of the concepts introduced from prior
lessons
also you should have already watched
part 1 of this lesson this video is a
direct continuation
in part 1 of this lesson we illustrated
everything hosts do to communicate to
other hosts on the same network
in this video we're going to add a
router and we're going to show you
everything a host does
when trying to communicate with another
host on a foreign network
we're going to use this topology to show
you everything host a does
to send data to host c both of our hosts
and our router has a mac address and ip
addresses
and once again i'm only showing you the
short version of the mac address to
conserve
screen real estate this slash 24 you see
on all of them
is simply another way of representing
this subnet mask
and again the subnet mask simply defines
how big a particular network
is all of this is a function of
subnetting and once again we are not
going to be covering subnetting in this
module
in the first part of this lesson we
mentioned that anything with an ip
address has an arp cache
well host a the router and host c all
have an ip addresses so all three of
them
have an arp cache but this video we're
going to be focusing on host a so we're
only going to show you host ace arp
cache and we're going to show you its
functionality as host a
has some data to send to host c host a
already knows
host c's ip address we mentioned some
reasons for this
in the last video but essentially that
ip address could have been provided by
the user
or provided by the application that is
creating this data that is meant to be
sent to host
c one way or another host a already
knows the ip address it's trying to
speak to you
but in particular host a knows that that
ip address is on a foreign network
it's going to know this by looking at
its own ip address and subnet mask
and comparing it with a target ip
address that's how it's going to know
this ip address is on a foreign network
now once again
all of that is done through the magic of
subnetting and if you're interested in
knowing how that works check out the
videos at subnet ipv4.com
but in so far as this video go ahead and
take my word for it that host c's ip
address is indeed on a different network
than host a's ip address
either way since host a knows the ip
address it's trying to speak to
host a is able to create a layer 3
header identifying the two endpoints of
this communication
that layer 3 header is going to have a
source ip address of host a
and a destination ip address of host c
next host a needs to create a layer 2
header the purpose of the layer 2 header
is going to be to take the packet from
hop
to hop in this case since the target we
are trying to speak to is on a foreign
network
our next hop is going to be the router
meaning the purpose of this layer 2
header is going to be to get this packet
to the router the problem is at this
point in time
since host a's arp cache is empty host a
is unable to complete the layer 2 header
that'll take this packet to the router
host a
just like before is going to have to use
arp to resolve the mac address of the
router
but there's something we have to mention
how does host a
even know the router's ip address well
the answer to that is that the router's
ip address is already configured on host
a
as host a's default gateway
when you connect a computer to the
internet there are three things that you
have to configure
an ip address a subnet mask and a
default
gateway on a windows computer if you
type ipconfig into the command prompt
you'll see all three of these things
listed here i'm showing you
exactly what it might look like on host
a itself notice the ip address matches
host a's ip address
the mask is a slash 24 mask and in
particular the default gateway is
highlighting the ip address of our
router
that's how host a knows the router's ip
address
because it's been configured as its
default gateway
that's the ip address that host a is
going to have to resolve with arp
so just like we mentioned in the last
lesson host a is going to shoot out an
arp request
that arp request is going to ask for the
mac address that correlates to a
particular ip address
and again host a is going to include its
own arp mapping
in the request itself that arp request
is going to get to the router and then
the router is going to generate a
response
that response is going to include the
mapping that host a was interested in
learning
the ip address 10.1.1.1 maps to the mac
address e5e5
and when that arp response arrives on
host a
host a is able to populate its arp cache
with the mapping for its default gateway
it can then use this mapping to complete
the layer 2 header
the destination mac address for that
layer 2 header is going to be
the router's mac address this will allow
this header to handle hop to hop
delivery
of this packet and now
that data can finally be sent across the
wire
where it will be received by the router
upon receiving this the router is going
to discard the layer 2 header
the whole purpose of that header was to
get this packet
from host a's nick to the router snake
that header did that successfully and
can go and retire happily in header
heaven
and at this point the router is going to
take over presumably
the router is going to add layer 2
headers as necessary
to get the packet across the next hop
whether that hop is directly to host c
or whether that hop is across multiple
routers on the internet
one way or another from host a's
perspective the job is done
it did everything it needed to do to get
the data
to the router and from here we're just
going to hope that the router can do the
rest of the work to get it
all the way to host c now
i want to highlight something important
for you this arp entry
that host a resolved in order to get the
packet
to the router can be reused to speak to
any host
in foreign networks so let me show you
what i mean by that
so here again is the layer 3 and layer 2
header that were used to get this data
to host c
and let's go ahead and add other hosts
to our topology
let's say now host a is going to be
trying to speak to host
d well since this is a new endpoint
we're going to need
a new layer 3 header this layer 3 header
is going to have a source ip address of
host a
as before and of course a destination ip
address of host d
but notice the layer 2 header doesn't
change host a's first hop is
always going to be to the first router
and therefore
the source mac address and destination
mac address are always going to identify
those two nics so this arp
process that we had to go through to
resolve the router's ip address really
only needs to happen
once once host a knows the router's mac
address
it can reuse that mac address to speak
to any host on a foreign
network in all cases the layer two
header is going to look
the same so it's important to understand
this arp process is very crucial to how
data moves through a network
it's actually the first step that host a
or
any host takes when it's trying to send
data on a network
is to determine if the target ip it's
trying to speak to is on
my own network the local network or a
foreign network
if it's on a foreign network like we
just showed you arp is going to try and
resolve
the default gateways ip address
and if it's trying to speak to something
on the local network arp is going to try
and resolve the target ip address
directly
that's actually the illustration we
showed you in part one of this lesson
we showed you that for host a to speak
to host b
the arp entry that host a had to create
was for host b's ip address
directly this is what allowed host a to
create this
layer 2 and layer 3 header to get data
from host a to host b
and that wraps up our lesson on
everything hosts do
to speak on the internet the key points
of this lesson was understanding what
hosts do when speaking to other hosts on
a local network
or other hosts on a foreign network and
in particular
understanding how the layer 3 and the
layer 2 headers are populated to get the
data to the other host
moreover you should also understand
arp's role in this entire process
if you understand these three key points
and you followed all the concepts in
this lesson
then you now know everything hosts do to
communicate with other hosts in the same
network
regardless of how they are actually
connected and everything hosts do to
communicate to other hosts in foreign
networks
regardless of how they are connected in
the next lesson we're going to take the
focus away from hosts and instead
look at these devices right here we're
going to unpack everything switches do
to facilitate communication within
a network and everything routers do
facilitate communication between
networks but that's it for this lesson
i hope you enjoyed this video i want to
thank you for watching and we'll see you
in the next one hey youtube i hope you
enjoyed that free lesson for my new
course on networking 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
let me know in the comments below what
subjects you want included in this
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 in the next one
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