Everything Routers do - Part 2 - How Routers forward Packets - Networking Fundamentals - Lesson 5

Summary

This video lesson delves into the workings of routers, explaining how they use IP and MAC addresses, routing tables, and ARP tables to direct data across networks. It illustrates packet forwarding through routers using routing tables and ARP for MAC address resolution. The tutorial covers how hosts communicate with routers and other hosts, detailing the process from initial ARP requests to final data delivery, emphasizing the foundational concepts of networking.

Takeaways

• 🌐 **Routers and IP/MAC Addresses**: Routers have IP and MAC addresses in each network they are connected to.
• 📖 **Routing Tables**: Routers maintain routing tables, which are maps of networks they know about, and can be populated in three ways.
• 🔍 **ARP Tables**: ARP tables map IP addresses to MAC addresses, starting empty and populating dynamically as needed.
• 💡 **ARP Process**: If a router doesn't have an ARP entry, it uses the Address Resolution Protocol to find the necessary MAC address.
• 🔄 **Packet Forwarding**: Routers use routing tables to determine where to send packets and ARP tables to find MAC addresses for direct network nodes.
• 🔧 **Layer 2 Headers**: Routers construct Layer 2 headers to move packets from one MAC address to another within a network.
• 🔄 **Layer 3 Headers**: Hosts create Layer 3 headers with source and destination IP addresses to send data to foreign networks.
• 🔍 **ARP Requests**: Hosts send ARP requests to find the MAC address of their default gateway or next hop router.
• 📚 **Learning Process**: Routers learn ARP mappings from incoming ARP requests, which they can then use for future packet forwarding.
• 🔁 **Return Traffic**: The process of sending packets back to the original host is faster as ARP entries have already been populated.
• 🌟 **Internet as a Network of Routers**: The internet functions as a series of routers passing packets between each other using routing and ARP tables.

Q & A

What is the main focus of this lesson on routers?

The lesson focuses on explaining how routers use their routing tables and ARP tables to forward packets across networks, including detailed steps on how packets travel between devices.

What was covered in part one of this lesson that this video builds upon?

Part one discussed how routers maintain routing tables, which are populated using three methods, and how routers use these tables to route traffic between networks.

What is the purpose of an ARP table in a router?

An ARP table maps IP addresses (Layer 3 addresses) to MAC addresses (Layer 2 addresses). It helps routers deliver packets to the correct network nodes by resolving their MAC addresses.

How are ARP tables populated compared to routing tables?

Unlike routing tables, which are populated ahead of time, ARP tables start empty and are dynamically populated as traffic flows through the network using the address resolution process.

What does Host A need to send a packet to Host C, and what happens if it doesn't know the router’s MAC address?

Host A needs to construct a Layer 2 header with the router’s MAC address. If it doesn't know the MAC address, it sends an ARP request to resolve the router's MAC address.

What happens when a router receives a packet with a Layer 2 header?

The router discards the Layer 2 header (since it only helped deliver the packet to that router) and looks at the Layer 3 header to determine where to forward the packet next, based on its routing table.

What occurs if a router doesn’t have an ARP entry for the next hop?

If a router doesn’t have the MAC address for the next hop, it sends an ARP request to resolve the Layer 2 address, allowing it to construct the necessary Layer 2 header for the next transmission.

How does the return traffic from Host C to Host A differ from the original transmission?

The return traffic from Host C to Host A is faster because the necessary ARP entries have already been populated, allowing routers to forward the packet without additional ARP requests.

What would happen if there were more routers between Host A and Host C?

If more routers were involved, each router would follow the same process of checking its routing table for the next hop and using ARP to resolve the MAC address, if necessary.

What is the key takeaway from this lesson on how routers function?

The key takeaway is understanding the systematic process routers use—leveraging routing and ARP tables to forward packets across networks and how they dynamically handle ARP requests when necessary.

Outlines

00:00

🌐 Understanding Routers and ARP Tables

This paragraph introduces part two of lesson five, focusing on the role of routers in networking. It builds upon the previous lesson, explaining that routers have both IP and MAC addresses and maintain routing tables. The lesson also introduces ARP tables, which map IP addresses to MAC addresses for nodes in directly connected networks. The dynamic nature of ARP tables is highlighted, contrasting with the pre-populated routing tables. An example using Router 1 and Router 2 illustrates how these tables are used to route packets from Host A to Host C. The process involves Host A sending an ARP request to find Router 1's MAC address, allowing it to construct a Layer 2 header. The importance of ARP in facilitating communication between devices on different networks is emphasized.

05:02

🔄 Routers' Role in Packet Forwarding

The second paragraph delves into the mechanics of how routers use Layer 2 and Layer 3 headers to forward packets. It describes the process of Router 1 receiving a packet from Host A and using its routing table to determine the next hop, which is Router 2. The necessity for Router 1 to perform an ARP request to learn Router 2's MAC address is explained. Once the MAC address is known, Router 1 can construct a Layer 2 header to forward the packet. The process is repeated at Router 2, which also needs to perform an ARP request to learn Host C's MAC address before forwarding the packet. The summary underscores the importance of ARP in enabling routers to forward packets to their correct destinations.

10:05

🔙 Efficient Packet Routing in Networks

The final paragraph discusses the return path for packets from Host C to Host A, highlighting how the process is expedited once ARP entries are populated. It explains that Host C, knowing the MAC address of Router 2 from previous ARP exchanges, can construct a Layer 2 header to send packets back. The paragraph reiterates the process of routers using routing tables and ARP tables to determine next hops and construct necessary headers for packet forwarding. It concludes by emphasizing that these principles apply regardless of the number of routers in the path, illustrating the fundamental nature of packet routing across complex networks like the internet. The paragraph ends with a call to action for viewers to support the creation of a full networking course by sharing the content and engaging with the community.

Keywords

💡Routers

Routers are networking devices that forward data packets between networks. In the video, routers are discussed as essential components of internet infrastructure, with each router having an IP address and a MAC address in each network they are connected to. The script illustrates how routers use their routing tables to determine the best path for data to travel.

💡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. In the script, it is mentioned that routers have an IP address in each network they are connected to, which is crucial for routing data packets.

💡MAC Address

A MAC address is a unique identifier assigned to network interfaces for communications on the physical network segment. The video script explains that routers maintain a mapping of IP addresses to MAC addresses for devices in directly connected networks, which is vital for the physical delivery of packets.

💡Routing Tables

Routing tables are databases in routers that maintain a map of every network each router knows about. The video script describes how routers populate their routing tables using three methods and use these tables to determine where to send packets next.

💡ARP Tables

ARP tables are used to map IP addresses to their corresponding MAC addresses. The script explains that unlike routing tables, ARP tables start out empty and are populated dynamically as traffic flows through the network, which is essential for the physical delivery of packets to the next hop.

💡Address Resolution Protocol (ARP)

ARP is a protocol used for discovering the MAC address of a device on a local network. The video script uses ARP to illustrate how routers and hosts learn each other's MAC addresses when they don't already know them, which is necessary for constructing the Layer 2 header of a packet.

💡Layer 2 Header

The Layer 2 header is part of a data packet that contains the MAC addresses necessary for moving the packet from one device to another on the same network. The video script explains how hosts and routers construct Layer 2 headers once they know the MAC address of the next hop.

💡Layer 3 Header

The Layer 3 header contains the IP addresses used for routing the packet between networks. The script describes how hosts create Layer 3 headers with source and destination IP addresses to send data to foreign networks via their default gateway.

💡Default Gateway

A default gateway is the node on a network that is responsible for passing information to other networks. In the video, hosts send packets destined for foreign networks to their default gateway, which is typically a router like R1 in the script.

💡Next Hop

The next hop is the immediate destination of a packet as it travels towards its final destination. The video script discusses how routers determine the next hop for a packet by looking up the destination IP address in their routing tables.

💡Packet Forwarding

Packet forwarding is the process of transmitting data packets from one network to another. The video script describes the detailed process of how routers use their routing and ARP tables to forward packets from one network to another, including the steps taken when the MAC address is unknown.

Highlights

Introduction to part two of lesson five on routers and networking fundamentals.

Continuation from part one, focusing on routers' IP and MAC addresses in each network.

Explanation of routing tables as a map of networks known to each router.

Three methods for populating routes in a routing table.

Demonstration of Router 1 and Router 2's routing tables.

Introduction to ARP tables and their function in mapping IP to MAC addresses.

ARP tables start empty and populate dynamically as traffic flows through the network.

Process of how a packet is sent from Host A to Router 1 using ARP.

Router 1's use of its routing table to determine the next hop for a packet.

Router 1's need to perform ARP to find the MAC address of Router 2.

Router 2's process for receiving a packet and looking up the destination IP address in its routing table.

Router 2's need to perform ARP to find the MAC address of Host C.

Detailed step-by-step of data transfer from Host A to Host C.

Explanation of how response data travels back from Host C to Host A.

Highlight of how ARP entries are already populated for quicker return of data.

Final thoughts on the process every router follows using routing and ARP tables.

Invitation to join the community on Discord for further learning.

Call to action for viewers to support the creation of a full networking fundamentals course.

Transcripts

hello welcome to part two of lesson five

where we will continue our discussion of

routers this is another lesson from my

new course on networking fundamentals

which will teach you everything you need

to understand how data flows through the

internet

in this lesson we'll continue right

where we left off from the last video

in part one of this lesson we used this

topology to illustrate that routers

have an ip address and a mac address in

each network that they are connected to

we also learned that routers maintain

routing tables which is a map of every

network that each router knows about

and that routes in that routing table

can be populated in one of these three

ways

we showed you all this by showing you

router 1 and router 2's routing tables

and showed you how each router populates

its routing table using one of these

three methods

now all this was covered in great detail

in part one of this video

if you haven't seen that video yet go

ahead and pause this video and watch

that one first

as this video is simply going to build

upon these concepts

now something we didn't mention in part

one of this video

is that not only do routers have routing

tables

but routers also have arp tables arp if

you recall is a mapping of a layer 3

address like an ip address to a layer 2

address like a mac address

now we discussed arp in lesson 3 of this

series in that lesson i told you that

everything with an ip address has an arp

table

well just like we just mentioned routers

also have ip

addresses therefore they also have arp

tables

in each case the router's arp tables are

going to maintain a mapping of ip

addresses to mac addresses

of all the nodes in directly connected

networks

take a look at r1's arp table right here

notice

in the 1044 network there is only one

other node other than the router

and router 1 is going to have an arp

entry correlating to the ip address and

mac of host

a for this network there are two

other nodes in that network other than

router 1 and router 1 is going to have

an arp entry for each of them

this one belonging to host b and this

one belonging to

router 2. and router 2 is the same way

there's only one other node in this

network that's host c

and there's the arp mapping for host c

and then in this network over here there

are two

other nodes other than r2 and you have a

mapping for each of them

in router 2's arp table now the thing

about arp tables

is that unlike routing tables arp tables

actually start out empty

they will get populated dynamically as

needed as traffic is flowing through

this network

that's different from the routing table

the routing table has to be populated

ahead of time

recall that if a router receives a

packet that it doesn't know how to

deliver

the router is just going to drop that

packet but with arp

if the router doesn't have an arp entry

it needs it can simply do

the address resolution process to figure

out what it needs

we're going to show you how router 1 and

router 2 use their routing tables and

r tables in order to get a packet all

the way from host a through both of

these routers to host c

it's going to start with host a having

some data it needs to send to host c

host a knows the ip address it's trying

to speak to so it's able to construct a

layer 3 header that's going to include a

source ip address of host a

and a destination ip address of host c

now we discussed some reasons of why

host a might already know the ip address

is trying to speak to

we reviewed that back in lesson three

feel free to check that out if you want

a refresher

either way host a can look at the

destination ip address and compare that

to its own ip address

to determine that what it's trying to

speak to is on a foreign network

which means this packet must be sent to

host a's default gateway

which in this case is r1 the issue

is if this is the first packet that host

a has sent host a

doesn't know router 1's mac address and

therefore

cannot construct the necessary layer 2

header which would take this packet

from host a's nick to router 1 snake

so before host a can send the packet

host a is going to have to perform arp

so host a is going to send an arp

request for the ip address of r1

100 44.1 that arp request is going to

ask for the mac address

for whoever owns the ip1044.1

notice in the arp request host a is

going to include its

own ip to mac address mapping this arp

request will be sent across the wire

where it'll be received by router 1 and

when router 1 receives the arp request

router 1 will be able to learn the arp

mapping of the sender of the arp request

recall that the sender included the arc

mapping of host a

and there's nothing preventing router 1

from simply adding

that arp entry to its arc table upon

receiving the arp request

narrator 1 needs to generate a response

that response will include the arp

mapping that host a was trying to

resolve

when that arp response gets back to host

a host a will populate its arp entry

with the mac address of its default

gateway and now

host a has everything it needs to

properly construct the layer 2 header

which will get this packet from host a

to the router

host a can now send all of this to the

router

when the router receives this packet the

first thing the router is going to do

is discard the layer 2 header remember

that header only existed to take that

packet from this mac address to this mac

address

it did so successfully so we can discard

that layer 2 header

now router 1 will look at the

destination ip address in its routing

table it's going to try and find a match

to determine where to send this packet

next

it'll see that there's a match right

here which will tell router 1 that the

next hop for this particular packet

is the ip address 5 100552

which is router 2's ip address

now router 1 needs to construct a layer

2 header

which will take this packet from this

mac address to this mac address

the problem is at this point router 1

does not have an

arp entry for the ip address 10055.2

therefore router 1 cannot construct the

layer 2 header as necessary

which means router 1 is going to have to

perform the address resolution

protocol router 1 will send an arp

request

for the ip address 10.0.55.2

just like before the arp request

includes the senders

arp mapping this is going to allow r2

to learn the art mapping of the sender

of that arp request

upon receiving the arp request r2 will

learn that something with the ip address

1055.1

has the mac address ee3 now

r2 will generate an arp response this

arp response

is the arp mapping that router 1 was

trying to discover

when router 1 receives this arp response

router 1 will be able to complete its

arp

entry and therefore able to create a

layer 2 header

which will get the packet through to the

next hop

now this packet can be sent from this

nick

to this neck once it gets there

router 2 is going to receive the packet

and discard the layer 2 header

again that header only existed to go

from here to here

then router 2 will look at the

destination ip address in the routing

table to try and determine what to do

with that packet next

it'll find a match right here indicating

that this packet needs to be delivered

out the left interface since router 2

is now delivering this packet through a

directly connected route

it knows that this is actually the final

hop for this particular packet

because the destination exists in a

directly connected network

so router 2 will need to construct a

layer 2 header which will take

this packet from router 2's nick to host

seesnake

but just like before router 2 currently

does not know

host c's mac address and therefore

cannot create the layer 2 header

router 2 is going to have to send an arp

request to resolve the layer 2 address

for host c

this will follow the same process we've

already outlined

host c will learn the mac address

mapping for router 2

by receiving the arp request from router

2.

this will be useful for the return

traffic back to host a

then host c will generate an arp

response this arp response is going to

include

the arp mapping that router 2 was trying

to resolve

once router 2 receives the arp response

it'll populate its arp

table with the appropriate mac address

and then he can finally create the

appropriate layer 2 header

to get the packet to host c

when host c receives this packet it's

going to discard the layer 2 header

again this header's purpose was simply

to take the packet from this nick to

that nick

then ho c is going to discard the layer

3 header the purpose of that header was

simply to take

the data from this ip address to this ip

address

and finally host c will process the data

and that is every single step that needs

to happen

in order to get data from post a to host

c

next i'm going to show you what needs to

happen in order to get a response from

host c

back to host a but you'll notice the way

back will go much quicker

because all of the necessary arp entries

have already been populated

just like before it's going to start

with host c having some data to go to

host a

now these routers aren't going to know

that this is actually response data

from the perspective of these routers

this is just a bunch of ones and zeros

host c knows this data needs to get to

host a so it'll create

a layer 3 header with a source ip

address of host csip

address and a destination appears a host

a's ip address

and since host c knows that the

destination ip address is on a foreign

network

host c knows that this packet needs to

go to its default gateway

which in this case is router 2. and

since host c

already has the art mapping for router 2

it can create

the layer 2 header necessary to get this

packet

from this nic to this nic remember

that host c learned the mac address of

router 2 when router 2

asks for host c's arp entry either way

that's what's going to occur to get this

packet sent to r2

once r2 receives the packet it'll

discard the layer 2 header

then look up the destination ip address

in its routing table to determine what

to do with this packet next

it'll find a match for this ip address

with this route right here

which is going to tell router 2 that the

next hop of this particular packet is

the ip address

10055.1 which is rather once i p address

and since router 2 already has an arp

mapping for

router 1 router 2 can instantly create

the necessary layer 2 header

to get the packet across this hop so

with that layer 2 header created

router 2 can now send the packet to

router 1.

once it arrives to router 1 again router

1 will discard the layer 2 header

and then look up the destination ip

address in its routing table

it'll find a match for this route right

here which tells router 1 that this

particular packet needs to be delivered

out the right interface to its final hop

and since router 1 has the arp mapping

for the ip address 1044.9

router 1 is able to construct a layer 2

header

which will get the packet across to its

final hop

this will then allow router 1 to send

this packet to host

a once host a receives the packet host a

will discard the layer 2 header

then host a will discard the layer 3

header and finally host a will process

the response data

and that is everything that occurs to

get the response data

from host c all the way back to host a

and that wraps up the major ideas i

wanted to communicate to you

in this lesson the main takeaways are

understanding how

the routers are going to use their

routing tables and their arp tables in

order to forward packets across a

network

but before i let you go there's still

one last thought i want to leave you

with

everything we did between router 2 and

router 1 in order to

move data from host c to host a would

repeat for any amount of routers in the

path

in each case every router in the path

would look up the destination ip address

in

their routing table to determine the

next hop ip address

and then construct a layer 2 header with

the appropriate mac addresses

to get the packet across to the next hop

if for whatever reason

a router doesn't know the destination

mac address that router will perform arp

as necessary these steps are the same

steps we just illustrated to get a

packet from router 2 to router 1.

but those same steps would also occur if

there were many

other routers in the path between router

2 and router 1.

what we just illustrated essentially

showed you how a packet gets from host c

all the way through to host a whether

host a

is on the other side of two routers or

five routers

or the other side of the internet in

fact

the internet is really nothing more than

just a series of routers that are

handing packets off between

other routers and with that

we finally wrap up our discussion on

routers

the main takeaway is understanding the

process that every router will follow

and how the routers will use their

routing table and their arp tables to

move packets along

i hope you enjoyed this video i want to

thank you for watching

and we'll see you in the next one hey

youtube

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