Basic Routing Concepts

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Del curso dictado por Google

The Bits and Bytes of Computer Networking

6056 calificaciones

Google

The Bits and Bytes of Computer Networking

6056 calificaciones

Curso 2 de 5 en SpecializationGoogle IT Support Professional Certificate

This course is designed to provide a full overview of computer networking. We’ll cover everything from the fundamentals of modern networking technologies and protocols to an overview of the cloud to practical applications and network troubleshooting. We’ll wrap up by covering how this information might show up in a job interview and giving you a few tips for troubleshooting on the spot. By the end of this course, you’ll be able to: ● describe computer networks in terms of a five-layer model. ● understand all of the standard protocols involved with TCP/IP communications. ● grasp powerful network troubleshooting tools and techniques. ● learn network services like DNS and DHCP that help make computer networks run. ● understand cloud computing, everything as a service, and cloud storage.

De la lección

The Network Layer

In the second week of this course, we'll explore the network layer in more depth. We'll learn about the IP addressing scheme and how subnetting works. We'll explore how encapsulation works and how protocols such as ARP allow different layers of the network to communicate. We'll also cover the basics of routing, routing protocols, and how the Internet works. By the end of this module, you'll be able to describe the IP addressing scheme, understand how subnetting works, perform binary math to describe subnets, and understand how the Internet works.

Basic Routing Concepts8:22

Routing Tables3:35

Interior Gateway Protocols5:46

Exterior Gateway Protocols3:10

Non-Routable Address Space3:19

Alex Good Story1:30

Conoce a los instructores

Google

The Internet is an incredibly impressive technological achievement.

It meshes together millions of individual networks and

allows communications to flow between them.

From almost anywhere in the world, you can now access data from almost anywhere else.

Often in just fractions of a second.

The way communications happen across all these networks,

allowing you to access data from the other side of the planet, is through routing.

By the end of this lesson, you'll be able to describe the basics of routing and

how routing tables work.

You'll be able to define some of the major routing protocols and

what they do and identifying non-routable address space and how it's used.

You'll also gain an understanding of the RFC system and

how it made the Internet what it is today.

All of these are very important skills in order to troubleshoot

the networking issues you might run into as an IT support specialist.

Routing is one of those things that is very simple and very complex.

At a very high level, what routing is and

how routers work is actually pretty simple.

But underneath the hood, routing is a very complex and

technologically advanced topic.

Entire books have been written about the topic.

Today most intensive routing issues are almost exclusively handled by ISPs and

only the largest of companies.

We'll arm you with the basic overview of routing

to give you a well rounded networking education,

since it's an important topic to understand no matter what.

But by no means will our coverage be exhaustive.

From a very basic standpoint, a router is a network device

that forwards traffic depending on the destination address of that traffic.

A router is a device that has at least two network interfaces,

since it has to be connected to two networks to do its job.

Basic routing has just a few steps.

One, a router receives a packet of data on one of its interfaces.

Two, the router examines the destination IP of this packet.

Three, the router then looks up the destination network of this IP

in its routing table.

Four, the router forwards that out though the interface that's closest to

the remote network.

As determined by additional info within the routing table.

These steps are repeated as often as needed until the traffic

reaches its destination.

Let's imagine a router connected to two networks.

We'll call the first network, Network A and give it

an address space of 192.168.1.0/24.

We'll call the second network, Network B and

give it an address space of 10.0.0.0/24.

The router has an interface on each network.

On Network A, it has an IP of 192.168.1.1 and

on Network B, it has an IP of 10.0.254.

Remember, IP addresses belong to networks, not individual nodes on a network.

A computer on Network A with an IP address of 192.168.1.100 sends a packet

to the address 10.0.0.10.

This computer knows that 10.0.0.10 isn't on its local subnet.

So it sends this packet to the MAC address of its gateway, the router.

The router's interface on Network A receives the packet

because it sees that destination MAC address belongs to it.

The router then trips away the Data Link Layer encapsulation,

leaving the network layer content, the IP datagram.

Now, the router can directly inspect the IP datagram header for

the destination IP field.

It finds the destination IP of 10.0.0.10.

The router looks at it's routing table and sees that Network B, or

the 10.0.0.0/24 network, is the correct network for the destination IP.

It also sees that, this network is only one hop away.

In fact, since it's directly connected, the router even has the MAC address for

this IP in its ARP table.

Next, the router needs to form a new packet to forward along to Network B.

It takes all of the data from the first IP datagram and duplicates it.

But decrements the TTL field by one and calculates a new checksum.

Then it encapsulates this new IP datagram inside of a new Ethernet frame.

This time, it sets its own MAC address of the interface on network B

as the source MAC address.

Since it has the MAC address of 10.0.0.10 in its ARP table,

it sets that as the destination MAC address.

Lastly, the packet is sent out of its interface on Network B and

the data finally gets delivered to the node living at 10.0.0.10.

That's a pretty basic example of how routing works, but

let's make it a little more complicated and introduce a third network.

Everything else is still the same.

We have network A whose address space is 192.168.1.0/24.

We have network B whose address space is 10.0.0/24.

The router that bridges these two networks still has the IPs of

192.168.1.1 on Network A and 10.0.0.254 on Network B.

But let's introduce a third network, Network C.

It has an address space of 172.16.1.0/23.

There is a second router connecting network B and network C.

It's interface on network B has an IP of 10.0.0.1 and

its interface on Network C has an IP of 172.16.1.1.

This time around our computer at 192.168.1.100

wants to send some data to the computer that has an IP of 172.16.1.100.

We'll skip the Data Link Layer stuff, but

remember that it's still happening, of course.

The computer at 192.168.1.100 knows that 172.16.1.100 is not on its local network,

so it sends a packet to its gateway, the router between Network A and Network B.

Again, the router inspects the content of this packet.

It sees a destination address of 172.16.1.100 and

through a lookup of its routing table, it knows that the quickest

way to get to the 172.16.1.0/23 network is via another router.

With an IP of 10.0.0.1.

The router decrements the TTL field and

sends it along to the router of 10.0.0.1.

This router then goes through the motions,

knows that the destination IP of 172.16.1.100 is directly connected and

forwards the packet to its final destination.

That's the basics of routing.

The only difference between our examples and

how things work on the Internet is scale.

Routers are usually connected to many more than just two networks.

Very often, your traffic may have to cross a dozen routers before it reaches

its final destination.

And finally, in order to protect against breakages,

core Internet routers are typically connected in a mesh,

meaning that there might be many different paths for a packet to take.

Still, the concepts are all the same.

Routers inspect the destination IP, look at the routing table to determine which

path is the quickest and forward the packet along the path.

This happens over and over.

Every single pocket making up every single bit of traffic all over the Internet

at all times.

Pretty cool stuff?

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