Free CCNA | The Life of a Packet | Day 12 | CCNA 200-301 Complete Course

Summary

In this CCNA-focused video by Jeremy's IT Lab, viewers are taken through a comprehensive theoretical overview of a packet's journey across networks, without hands-on router or switch configuration. The video covers key concepts like ARP, encapsulation, and de-encapsulation, using a specific network topology to illustrate the process. It emphasizes the importance of understanding how packets move through different devices, including routers and switches, and the role of MAC addresses in this process. The video is designed to consolidate prior learning and prepare viewers for their CCNA exam, with a quiz at the end to test comprehension.

Takeaways

• 😀 This video is a theoretical overview rather than a practical demonstration, focusing on the conceptual understanding of packet transmission across networks.
• 🔄 The video covers the entire process of sending a packet to a remote destination, including ARP, encapsulation, and de-encapsulation.
• 📚 It is designed to reinforce and consolidate knowledge for the CCNA exam, assuming prior coverage of most topics.
• 🖥 The network topology used is the same as in a previous video on static routing, involving devices like PC1, R1, R2, R4, and PC4.
• 📍 MAC addresses are introduced and used to illustrate how devices communicate at the data link layer, with examples provided for each device.
• 🛰 The video explains how a packet is forwarded from PC1 to PC4 via the routers R1, R2, and R4, detailing the ARP process and routing decisions.
• 🔄 It highlights that the original packet's IP header remains unchanged throughout its journey across the network.
• 🔍 The video emphasizes the role of switches in forwarding frames without modifying them, unlike routers which handle packet encapsulation and de-encapsulation.
• 📝 The script includes a quiz to test understanding, using the network topology and the packet's journey as a basis for questions.
• 💼 Supplementary materials such as a Packet Tracer lab are mentioned for further practice, focusing on packet analysis in simulation mode.
• 📢 The video concludes with an encouragement to subscribe, like, comment, and share, and mentions the availability of tips via various platforms.

Q & A

What is the purpose of the video mentioned in the script?

The purpose of the video is to provide a comprehensive understanding of the process a packet goes through when being sent across networks, specifically for the CCNA (Cisco Certified Network Associate) exam preparation.

Why does the video emphasize the importance of understanding the packet sending process?

The video emphasizes the importance of understanding the packet sending process to ensure viewers have a solid foundation for their CCNA studies and to help them put together all the pieces of knowledge learned previously.

What network topology is used in the video to demonstrate the packet sending process?

The network topology used in the video is the same one used in the static routing demonstration from day 11's video, where a packet is sent from PC1 in the 192.168.1.0/24 network to PC4 in the 192.168.4.0/24 network.

What is the role of ARP (Address Resolution Protocol) in the packet sending process as described in the video?

ARP is used by devices to discover the MAC address of the next hop device when the MAC address is not known. It is essential for devices to send packets to the correct next hop on the network.

How does the video simplify the representation of MAC addresses for the purpose of the demonstration?

To save space and simplify the demonstration, the video shortens the 12 hexadecimal character MAC addresses to 4 characters, using placeholders like '1111' for PC1 and 'AAAA' for R1's G0/2 interface.

What is the significance of each network device having a unique MAC address as mentioned in the video?

Each network device having a unique MAC address is significant because it allows for proper identification and communication between devices at the data link layer of the OSI model.

Why does the video choose to focus on the path from PC1 to PC4 via R1, R2, R4, and not via R3?

The video chooses to focus on the path from PC1 to PC4 via R1, R2, R4 to maintain consistency with the static routing demonstration from a previous video, although the path via R3 is also valid.

What is the difference between the packet sending process for the first transmission from PC1 to PC4 and the return path from PC4 to PC1?

The main difference is that during the return path from PC4 to PC1, there is no need for ARP requests and replies since the MAC addresses of the devices have already been learned during the initial transmission.

How do switches contribute to the packet sending process as described in the video?

Switches contribute to the packet sending process by forwarding frames to the correct interface based on the destination MAC address and learning the MAC addresses of devices as they receive frames.

What supplementary materials are mentioned at the end of the video to help viewers practice and reinforce their learning?

The video mentions a Packet Tracer lab where viewers can use Packet Tracer's 'simulation' mode to analyze a packet and test their knowledge and understanding.

Outlines

00:00

🌐 Introduction to Packet's Journey in Networking

Jeremy introduces a CCNA course video that focuses on the conceptual understanding of how packets travel across networks rather than practical configuration. The video aims to provide a comprehensive view of packet transmission, including processes like ARP, encapsulation, and de-encapsulation. It emphasizes the importance of understanding these processes for CCNA preparation. The network topology used is the same as in a previous video on static routing, involving a path from PC1 to PC4 via routers R1, R2, and R4. MAC addresses are introduced for a more detailed layer 2 analysis, and the video promises to connect previously learned concepts.

05:05

📨 The ARP Process and Frame Encapsulation

This section delves into the Address Resolution Protocol (ARP) process, explaining how PC1 uses ARP to discover the MAC address of its default gateway, R1. It details the construction and transmission of ARP request and reply frames, including the use of broadcast and unicast MAC addresses. The video clarifies the order of IP and MAC addresses in IPv4 headers versus Ethernet headers. It also describes how switches like SW1 learn MAC addresses and how routers like R1 handle packet forwarding after ARP resolution, including the process of re-encapsulating packets with the appropriate Ethernet headers for the next hop.

10:10

🔍 Deep Dive into Router's Role in Packet Forwarding

The video continues with an in-depth look at how routers forward packets. It explains the ARP process between R1 and R2, and then between R2 and R4, to discover each other's MAC addresses. The process of encapsulating the packet with the correct Ethernet header for the next hop is detailed, including the use of broadcast MAC addresses for initial ARP requests. The video also discusses how switches forward frames without modifying them and how routers update the Ethernet headers while keeping the original packet intact. It sets the stage for understanding the return path of a packet from PC4 to PC1, highlighting that ARP will not be needed for the return path due to pre-learned MAC addresses.

15:13

📝 Quiz and Supplementary Materials for Learning

The final part of the video script introduces a quiz to test the viewer's understanding of the packet's journey. It presents questions about the MAC and IP addresses at various stages of the packet's forwarding process from PC4 back to PC1. The quiz is designed to reinforce the concepts discussed throughout the video. Additionally, the video mentions supplementary materials, including a Packet Tracer lab for hands-on practice, and encourages viewers to create or edit flashcards for better retention of the learned material. The video concludes with a call to action for viewers to engage with the content by subscribing, liking, commenting, and sharing the video.

Keywords

💡CCNA

CCNA stands for Cisco Certified Network Associate. It is an associate-level certification program offered by Cisco Systems, focusing on the skills needed to install, configure, operate, and troubleshoot medium-sized routed and switched networks. In the context of the video, the course is designed to prepare viewers for the CCNA exam, providing practical knowledge and understanding of networking concepts, as demonstrated by the detailed discussion of packet transmission processes.

💡ARP (Address Resolution Protocol)

ARP is a communication protocol used to map an IP address to a physical (MAC) address on a local network. It is fundamental to network communication as it allows devices to identify the hardware addresses of other devices they need to communicate with. In the video, ARP is explained as a process used by PC1 to discover the MAC address of its default gateway (R1) before sending packets to a remote network.

💡Encapsulation

Encapsulation in networking refers to the process of wrapping data packets in protocol-specific headers and trailers, which provide control information to deliver the data to its destination. The video discusses encapsulation in the context of adding Ethernet headers to packets, which include the MAC addresses necessary for local network communication.

💡De-encapsulation

De-encapsulation is the reverse process of encapsulation, where the headers and trailers are removed from data packets as they reach their destination or are forwarded by network devices. The video explains that routers remove the Ethernet header before processing the packet and then re-encapsulate it with a new header for the next hop.

💡MAC address

A MAC address, or Media Access Control address, is a unique identifier assigned to network interfaces for communications on the physical network segment. The video script uses MAC addresses to illustrate how devices communicate at the data link layer, with examples like PC1 using ARP to find R1's MAC address.

💡IP address

An IP address is a numerical label assigned to devices participating in a computer network that uses the Internet Protocol for communication. The video emphasizes the importance of IP addresses in routing packets across networks, as the source and destination IP addresses remain constant throughout the packet's journey.

💡Routing table

A routing table is a set of rules that a router uses to determine where to forward packets based on their destination IP addresses. In the video, routing tables are mentioned as essential for routers to decide the next hop for packets, using the most specific match for the destination network.

💡Static routing

Static routing is a method of configuring a router's routing table with fixed routes that do not change unless manually altered by a network administrator. The video uses static routing as an example of how packets are sent from one network to another, with pre-configured paths that the packet follows.

💡Ethernet header

An Ethernet header is a part of the Ethernet frame format, which includes the destination and source MAC addresses, along with the type of protocol used in the data payload. The video script describes how Ethernet headers are used to encapsulate packets at the data link layer, with specific MAC addresses for local network communication.

💡Broadcast MAC address

A broadcast MAC address, often represented as all Fs (e.g., FF:FF:FF:FF:FF:FF), is used to send a message to all devices on a local network. The video explains that devices use the broadcast MAC address when they do not know the specific MAC address of the recipient, as seen when PC1 sends an ARP request.

💡Packet tracer

A packet tracer is a network simulation tool used to visualize and analyze network scenarios, including the behavior of packets as they traverse a network. The video mentions a packet tracer lab as supplementary material for viewers to practice and reinforce their understanding of packet transmission processes.

Highlights

Introduction to a free, complete course for the CCNA.

Encouragement for viewers to subscribe, like, comment, and share the video.

Explanation that the video will not be practical and will not involve configuring Cisco devices.

Emphasis on the importance of understanding the packet transmission process across networks.

Overview of the topics covered, including ARP, encapsulation, and de-encapsulation.

Assumption of pre-configured static routes for the packet's path demonstration.

Introduction of the network topology used for the packet transmission example.

Description of the packet's journey from PC1 to PC4 across different networks.

Discussion on the use of MAC addresses and their significance in network devices.

Explanation of the ARP process and its role in discovering the MAC address of the default gateway.

Detail on how switches learn and forward frames based on MAC addresses.

Process of routers using ARP to discover the MAC addresses of next-hop devices.

Description of how routers update Ethernet headers while forwarding packets.

Clarification that switches do not modify the original packet, only the Ethernet headers.

Discussion on the return path of a packet from PC4 to PC1 and the differences in the ARP process.

Introduction of a quiz to test understanding of the packet transmission process.

Announcement of supplementary materials, including a Packet Tracer lab for further practice.

Conclusion of the video with a call to action for viewers to engage with the content and support the channel.

Transcripts

Welcome to Jeremy’s IT Lab.

This is a free, complete course for the CCNA.

If you like these videos, please subscribe to follow along with the series.

Also, please like and leave a comment, and share the video to help spread this free series

of videos.

Thanks for your help.

This video, unlike the last one, is not going to be practical, meaning that you won’t

actually go on and configure a Cisco router or switch.

Also, most of the information in this video won’t be new, we’ve already covered most

of it in previous videos.

However, I decided to make this video because I think it’s very important to make sure

you have a good understanding of the complete process a packet goes through when being sent

across networks.

Hopefully this video will be a little shorter than the usual ones.

Let’s get started.

So, what will we cover in this video?

We’ll cover the entire process of sending a packet to a remote destination.

This will include things like ARP, encapsulation, de-encapsulation, etc.

Of course, there are different levels of depth we can go into when talking about this process,

and I won’t give unnecessary details that would only be expected of a CCNP or CCIE,

but in this video I hope to give you a solid understanding to get you ready for your CCNA.

My hope is that this video will help you put all of the pieces together that we learned

previously.

So, this is the life of a packet, the process a packet goes through when being sent to remote

networks.

Here’s the network topology we’ll use for this video.

If you watched day 11’s video, you should recognize this topology, as it’s the same

one we used to demonstrate static routing.

We’ll follow a packet being sent from PC1 in the 192.168.1.0/24 network, to PC4 in the

192.168.4.0/24 network.

Let’s assume we have pre-configured static routes on these devices, so that the packet

will follow the same path as in the static routing video, that is from PC1 to R1, R2,

R4, and then PC4.

This doesn’t have to be the path the packet takes, the path that goes via R3 instead of

R2 is valid too, but we’ll stick to the same path as last time.

Now, since we’re not just looking at Layer 3 in this video, let me add MAC addresses

for these devices.

I’ll use 1111 for PC1.

Now, as you know a MAC address is actually 12 hexadecimal characters, but just to save

space I’ll shorten them to 4.

R1’s G0/2 interface has a mac address of AAAA, and it’s G0/0 interface has a MAC

address of BBBB.

That’s something I didn’t mention before, each interface on a network device has a unique

MAC address.

Note that SW1’s interfaces also have MAC addresses, however for this video it’s not

necessary to know the MAC addresses of the switches so to avoid clutter, I’ll leave

them out of this diagram.

R2 has a MAC address of CCCC on its g0/0 interface, and DDDD on its G0/1 interface.

R4 has a MAC address of EEEE on its G0/1 interface and FFFE on its G0/2 interface.

I didn’t make it all Fs, because the MAC address of FFFF.FFFF.FFFF, 12 Fs, is the broadcast

MAC address, so just to avoid confusion I added that E on the end.

Finally, PC4 has a MAC address of 4444.

Okay, so PC1 wants to send some data to PC4, and its encapsulated in this IP header.

The source is 192.168.1.1, PC1’s IP address, and the destination is 192.168.4.1, PC4’s

IP address.

Now, because PC1’s IP address is in the 192.168.1.0/24 network, it notices that the

address 192.168.4.1 is in a different network, so it knows that it needs to send the packet

to its default gateway, which is R1, something we have already preconfigured.

However, in this example PC1 has not sent any traffic yet, so it needs to use ARP, the

address resolution protocol, something we covered in a previous video.

Let’s look at the ARP process once more.

So PC1 makes this ARP request packet.

The source IP is its own IP address and then destination is R1’s G0/2 interface, which

is the default gateway configured on PC1.

Next is the MAC addresses.

This is a minor point, but note that I put the source IP before the destination IP, but

the destination MAC before the source MAC.

That’s because, in the IPv4 header the source IP address comes first, but in the ethernet

header the destination MAC address comes first.

Anyway, just a minor point.

The destination MAC address is the broadcast MAC address of all Fs, because it

doesn’t know the MAC address of R1, so it will send the frame to all hosts on the network.

Finally the source MAC address is its own MAC address.

So, it sends the frame, which SW1 receives and broadcasts out of all its interfaces

except the one it received the frame on.

In this example, only PC1 and R1 are connected to SW1, so that means that SW1 will forward

the frame out of it’s G0/0 interface.

To translate the meaning of this frame into English, PC1 is saying ‘Hi 192.168.1.254.

What’s your MAC address?’.

Although I’m not going to really talk about the switches much in this video, note that

SW1 learns PC1’s MAC address on its G0/1 interface when the frame arrives on its G0/1

interface.

When this broadcast frame arrives on R1, it notices that the destination IP is its own

IP, so it creates this ARP reply frame to send back to PC1.

Although the ARP request message was broadcast, because R1 learned PC1’s IP and MAC addresses

from the ARP request message, the ARP reply can be sent unicast directly to PC1.

So, that’s what R1 does.

To translate this ARP reply message into english, basically it means Hi 192.168.1.1 This is 192.168.1.254.

My MAC address is aaaa.

Note that SW1 will learn R1’s MAC address from this message, when the frame arrives

on its G0/0 interface.

So, now PC1 knows the MAC address of its default gateway, so it encapsulates the packet with

this ethernet header.

Keep in mind, the original packet is not changed, the destination address remains PC4’s IP

address, NOT R1’s IP address.

Only at Layer 2 is the destination set to R1’s MAC address.

So, it sends the frame to R1.

R1 receives it, and removes the ethernet header.

It looks up the destination in its routing table.

The most specific match is this entry for the 192.168.4.0/24 network, which specifies

a next hop of 192.168.12.2.

So, R1 will have to encapsulate this packet with an Ethernet frame with the appropriate

MAC address for 192.168.12.2.

However, R1 doesn’t know R2’s MAC address yet. So,

how will it learn R2’s MAC address?

It will use ARP, of course.

The source IP address of this ARP request will be R1’s, and the destination will be

R2’s.

The destination MAC address is all Fs, the broadcast MAC address, because R1 doesn’t

know R2’s MAC address, and the source is bbbb, which is the MAC address of R1’s G0/0

interface.

So, it sends the arp request, and R2 receives it.

Basically, what this ARP request says is Hi 192.168.12.2, what’s your MAC address?

R2 receives the broadcast, and since the destination IP address matches its own IP address, it

makes this ARP reply to send to R1.

Once again, because it learned the IP and MAC addresses of R1 from the ARP request,

it doesn’t have to broadcast the frame.

So, it sends this ARP reply back, which basically says hi 192.168.12.1, this is 192.168.12.2.

My MAC address is cccc.

Okay, now R1 knows R2’s MAC address, so it can encapsulate the packet with an Ethernet

header, inserting R2’s MAC address in the destination field, and the MAC address of

R1’s G0/0 interface in the source field, and it sends it to R2.

After receiving the frame, R2 removes the Ethernet header.

R2 then looks up the destination IP address in its routing table, and the most specific

match is this one for 192.168.4.0/24, with a next hop of 192.168.24.4.

Although 192.168.24.0/24 is a connected network to R2, it doesn’t know the MAC address of

R4.

So, you know what’s next.

R2 will use ARP to discover R4’s MAC address.

R2 prepares this ARP request frame, using its own IP and MAC addresses for the source,

R4’s IP address as the destination, and the broadcast MAC address, and it forwards

it out of its G0/1 interface.

With this ARP request, R2 is saying ‘Hi 192.168.24.4.

What’s your MAC address?’

R4 receives the broadcast, and since the destination IP address is its own it creates this ARP

reply frame to send back to R2, once again it already knows R2’s IP and MAC addresses

because they were used as the source addresses for the ARP request.

It sends the unicast frame back to R2.

With this ARP reply, R4 is saying ‘Hi 192.168.24.2.

This is 192.168.24.4.

My MAC address is eeee.’

Now that R2 knows R4’s MAC address, it encapsulates PC1’s packet with an Ethernet header, with

a destination MAC address of eeee, which is R4’s g0/1 interface, and a source MAC address

of dddd, which is R2’s g0/1 interface.

R4 receives the frame and removes the Ethernet header.

It looks up 192.168.4.1 in its routing table, and the most specific match is this entry

for 192.168.4.0/24, which is directly connected via the G0/2 interface.

But, once again R4 doesn’t know PC4’s MAC address yet, so you know what it has to

do next.

It will use ARP to learn PC4’s MAC address.

It prepares this ARP request frame, again the source IP and MAC addresses are its own,

the destination IP address is PC4’s, and the destination MAC is the broadcast MAC address

of all F’s.

It sends this message out of its G0/2 interface, saying Hi 192.168.4.1, what’s your MAC address?

Note that SW4 will learn R4’s MAC address on its g0/0 interface from the source MAC

address field of this ethernet frame.

After PC4 receives the frame, it checks the destination IP address.

Since it is its own IP address, it will send an ARP reply.

The ARP reply will be unicast, using PC4’s IP and MAC addresses for the source and R4’s

IP and MAC addresses for the destination.

It sends the frame out of its network interface, saying ‘Hi 192.168.4.254.

This is 192.168.4.1.

My MAC address is 4444.’

Note that SW4 learns PC4’s MAC address when it arrives on its G0/1 interface.

Now that R4 knows PC4’s MAC address, it adds an ethernet header to the packet, using

its own MAC address on the G0/2 interface as the source address, and PC4’s MAC address

as the destination.

R4 sends the frame to PC4, and finally it has reached its destination.

Notice that the original packet hasn’t changed throughout the process.

It’s always used the same IP header with a source IP address of 192.168.1.1 and a destination

IP address of 192.168.4.1.

Also notice that the switches didn’t actually modify the frames at any point.

The switches forwarded the frames and learned the MAC addresses, but they don’t actually

de-encapsulate and then re-encapsulate the packet with a new ethernet header.

Okay, now let’s say PC4 sends a reply back to PC1, and we’ve configured static routes

on the routers so that the traffic follows the same path on the way back to PC1, going

via SW4, R4, R2, R1, SW1, and then reaching PC1.

What will be different?

First off, there will be one major difference.

Since these devices have already gone through the ARP process, there won’t be any need

for ARP requests and replies, the packet will simply be forwarded from device to device,

being de-encapsulated and then re-enapsulated as it is received by and then forwarded by

each router.

So, that’s it, just a basic walkthrough of how a packet is forwarded between routers

to pass it along to its final destination.

Now, as for today’s quiz, I’ll do something different than usual.

Instead of having multiple choice questions as usual, we’ll use this same diagram to

test your understanding.

Let’s get started with the quiz.

Here’s question 1.

PC4 sends a packet to PC1.

What is the destination MAC address when it is sent from PC4’s network interface?

Pause the video to think about your answer.

The answer is FFFE, which is the MAC address of R4’s G0/2 interface.

That’s because, to send the packet to PC1, which is in a remote network, PC4 must send

the packet to its default gateway, R4, first.

To do that, it encapsulates the packet with an ethernet header, with its default gateway’s

MAC address as the destination.

Okay, let’s go to question 2.

PC4 sends a packet to PC1.

What is the source MAC address when it is received on R1’s Gi0/0 interface?

Pause the video to think about your answer.

The answer is CCCC, which is the MAC address of R2’s G0/0 interface.

When R2 sends the packet to R1 en route to its destination, PC1, it encapsulates the

packet with an Ethernet header using its own MAC address as the source MAC address.

Okay, let’s go to question 3.

PC4 sends a packet to PC1.

What is the source MAC address when it is sent from SW1’s Gi0/1 interface?

Pause the video to think about your answer.

The answer is AAAA, which is the MAC address of R1’s G0/2 interface.

SW1 doesn’t alter the frame to use its own MAC address, it simply forwards the frame

out of the correct interface, or floods it if it doesn’t know the destination.

Let’s go to question 4.

PC4 sends a packet to PC1.

What is the destination IP address when it is sent from R4’s Gi0/1 interface?

Pause the video to think about your answer.

The answer is 192.168.1.1, which is the IP address of PC1.

Although each router modifies the source and destination MAC addresses in the Ethernet

header as it forwards the packet, they don’t modify the original packet itself, so the

destination IP address won’t change.

Since PC4 is sending the packet to PC1, that means the destination will be PC1’s IP address,

192.168.1.1.

Let’s go to question 5.

PC4 sends a packet to PC1.

What is the source IP address when it is received on R1’s Gi0/0 interface?

Pause the video to think about your answer.

The answer is 192.168.4.1, which is the IP address of PC4.

As I said in the last question, the original packet is not modified as the routers forward

it to its destination, so through the whole route the source IP address remains PC4’s

IP address, 192.168.4.1.

Okay, for this video there will once again be supplementary materials to help you practice

what you’ve learned.

There will be a packet tracer lab in which you use packet tracer’s ‘simulation’

mode to analyze a packet and test your knowledge and understanding. That will be the next video.

And that’s it, there won’t be a flashcard deck this video since there wasn’t actually

any new information in this video.

However, if there are some new points that you picked up in this video, feel free to

make your own flashcards.

Actually, even though I make flashcard decks for each video, I also think its a good idea

to make your own flashcards too, if there is anything else you want to remember.

You can also edit the flashcards I provide, or delete some flashcards if you think some

of them are not necessary.

The flashcards are just a tool to help you, so feel free to use them however you think

is best.

Okay, that’s all for today’s video.

Good luck with your studies.

Thank you for watching.

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with anyone else studying for the CCNA.

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