Everything Hosts do to speak on the Internet - Part 2 - Networking Fundamentals - Lesson 3

Summary

This 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

• 🌐 The course module covers how data flows through the internet, focusing on hosts and their communication.
• 🔑 It's essential to watch previous lessons for foundational concepts before diving into this video.
• 📡 Hosts use IP and MAC addresses to identify and communicate with each other on the network.
• 🛂 The subnet mask defines the size of a network, and subnetting is crucial for understanding how hosts determine the network they belong to.
• 📨 Hosts use ARP (Address Resolution Protocol) to resolve the MAC address of the next hop, which is necessary for sending data.
• 🚀 When communicating with a foreign network, hosts use their default gateway's IP address, which is configured as the router's IP address.
• 📬 ARP cache is vital for hosts to keep track of IP-to-MAC address mappings for efficient communication.
• 🔄 Once the ARP cache is populated, hosts can reuse the MAC address for the default gateway to communicate with any host on foreign networks.
• 📍 The lesson emphasizes the importance of understanding how layer 3 (IP) and layer 2 (MAC) headers are used in data transmission.
• 🔍 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

00:00

🌐 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.

05:00

🔄 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.

Keywords

💡Networking Fundamentals

Networking Fundamentals refers to the basic principles and concepts that underpin the operation of computer networks. In the context of the video, this term is central as the entire course is dedicated to teaching these foundational aspects. The video script discusses how data flows through the internet, which is a fundamental concept in networking.

💡Data Flow

Data Flow is the movement of data from one point to another within a network. The video script explains how hosts send data on the internet, which involves understanding the path data takes as it traverses networks, a fundamental aspect of networking.

💡Hosts

In networking, a host is a device connected to the network that is capable of sending or receiving data. The video script discusses the actions hosts take to send data, emphasizing the role of hosts in networking communications, particularly when communicating with other hosts on the same or different networks.

💡Internet Protocol (IP) Address

An IP address is a numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication. The script mentions that hosts and routers have IP addresses, which are essential for identifying devices on a network and facilitating communication.

💡MAC Address

A MAC address is a unique identifier assigned to network interfaces for communications on the physical network segment. The video explains that hosts and routers have MAC addresses, which are used at the data link layer of the OSI model to communicate within a local network.

💡Subnet Mask

A subnet mask is used to divide the IP address into the network address and the host address, allowing the system to determine whether the destination IP is on the local or foreign network. The script uses the subnet mask to illustrate how hosts determine the network size and the need to communicate through routers.

💡ARP Cache

The ARP cache is a table used to store the mapping of IP addresses to MAC addresses. The video script explains that hosts use the ARP cache to keep track of the MAC addresses associated with known IP addresses, which is crucial for sending data to the correct destination.

💡Layer 3 Header

A Layer 3 header, also known as an IP header, is part of the IP packet format and contains information necessary for routing the packet to its destination. The video script describes how hosts create a Layer 3 header with source and destination IP addresses to identify the endpoints of communication.

💡Layer 2 Header

A Layer 2 header, specifically an Ethernet header, is used to encapsulate the IP packet for transmission at the data link layer. The script explains that the Layer 2 header is used to get the packet from one hop to another, such as from a host to a router.

💡Router

A router is a networking device that forwards data packets between computer networks. The video script discusses the role of routers in facilitating communication between networks, particularly when a host is trying to communicate with a host on a foreign network.

💡ARP Request

An ARP request is a type of message used to request the MAC address associated with a given IP address. The video script illustrates how hosts use ARP requests to discover the MAC address of the router when the ARP cache does not already contain this information.

💡Default Gateway

The default gateway is the IP address of the router on a local network. The script explains that hosts are configured with a default gateway, which is used to send traffic destined for foreign networks, highlighting the importance of this configuration in network communication.

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