Ping: Internet Control Message Protocol

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The Bits and Bytes of Computer Networking

3082 ratings

Google

The Bits and Bytes of Computer Networking

3082 ratings

Curso 2 de 5 em Specialization Google IT Support Professional Certificate

This course is designed to provide a full overview of computer networking. In this course, we’ll cover everything from the fundamentals of modern networking technologies and protocols to practical applications and network troubleshooting. 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. Except as otherwise noted, the contents on this site are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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Troubleshooting and the Future of Networking

Congratulations, you've made it to the final week in the course! In the last week of this course, we'll explore the future of computer networking. We'll also cover the practical aspects of troubleshooting a network using popular operating systems. By the end of this module, you'll be able to detect and fix a lot of common network connectivity problems using tools available in Microsoft Windows, MacOS, and Linux operating systems.

Ping: Internet Control Message Protocol5:18

Testing Port Connectivity2:43

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When network problems come up,

the most common issue you'll run into is

the inability to establish a connection to something.

It could be a server you can't reach at all or a website that isn't loading.

Maybe you can only reach your resource on

your LAN and can't connect to anything on the internet.

Whatever the problem is,

being able to diagnose

connectivity issues is an important part of network troubleshooting.

By the end of this lesson,

you'll be able to use a number of

important troubleshooting tools to help resolve these issues.

When a network error occurs,

the device that detects it needs some way to

communicate this to the source of the problematic traffic.

It could be that a router doesn't know how to route to

a destination or that a certain port isn't reachable.

It could even be that the TTL of an IP datagram

expired and no further router hops will be attempted.

For all of these situations and more,

ICMP or internet control message protocol is used to communicate these issues.

ICMP is mainly used by router or remote hosts to

communicate while transmission has failed back to the origin of the transmission.

The makeup of an ICMP packet is pretty simple.

It has a header with a few fields and a data section that's used

by host to figure out which of their transmissions generated the error.

The first field is the type field,

eight bits long which specifies what type of message is being delivered.

Some examples are destination unreachable or time exceeded.

Immediately after this is the code field which

indicates a more specific reason for the message than just the type.

For example, of the destination unreachable type,

there are individual codes for things like destination network

unreachable and destination port unreachable.

After this is a 16 bit checksum

that works like every other checksum field we've covered so far.

Next up is a 32 bit field with an uninspired name, Rest of header.

You think they could come up with something a bit more

interesting but I can't really think of anything good.

So, who am I to judge?

Anyway, this field is optionally used

by some of the specific types and codes to send more data.

After this is the data payload for an ICMP packet.

The payload for an ICMP packet exists entirely so that

the recipient of the message knows which of

their transmissions caused the error being reported.

It contains the entire IP header and

the first eight bytes of the data payload section of the offending packet.

ICMP wasn't really developed for humans to interact with.

The point is so that these sorts of

error messages can be delivered between networked computers automatically.

But, there's also a specific tool and

two message types that are very useful to human operators.

This tool is called ping.

Some version of it exist on just about every operating system,

and has for a very long time.

Ping is a super simple program and

the basics are the same no matter which operating system you're using.

Ping lets you send a special type of ICMP message called an Echo Request.

An ICMP echo request essentially just ask the destination,

"Hey, are you there?"

If the destination is up and running and able to communicate on the network,

it will send back an ICMP echo reply message type.

You can invoke the ping command from the command line of any modern operating system.

In its most basic use,

you just type ping and a destination IP or a fully qualified domain name.

If you don't know how to use a command line in an operating system,

don't worry, you will soon.

We'll cover that in another course.

Output of the ping command is very

similar across each of the different operating systems.

Every line of output will generally display the address sending the ICMP echo reply,

and how long it took for the round trip communications.

It will also have the TTL remaining and how large the ICMP message is in bytes.

Once the command ends,

there will also be some statistics displayed

like percentage of packets transmitted and received,

the average round trip time,

and a couple of other things like that.

On Linux and Mac OS,

the ping command will run until it's

interrupted by an end user sending an interrupt event.

They do this by pressing the control key and the C key at the same time.

On Windows, ping defaults to only sending four echo requests.

In all environments, ping supports a number of command line flags

that let you change its behavior like the number of echo requests to send,

how large they should be,

and how quickly they should be sent.

Check out the documentation for your operating system to learn a little bit more.

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