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Welcome back. Let's talk about how

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data gets from one computer to another on the Internet.

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When your browser talks to some remote server, say www.si.umich.edu,

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it establishes a connection and then it sends some data,

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pieces of text or other formatted data,

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and each of those pieces of content gets chopped up into a bunch of packets.

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Each of those small packets gets sent out onto

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this big routing system that consists of a whole bunch of routers.

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Routers are just computers whose sole job

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is to receive these data packets and pass them on.

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Each packet will go along,

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goes from one router to the next.

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Each of those routers has a big lookup table that says,

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"If this packet that I've received is destined for some particular address,

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what's the next hop?

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What's the next router I should send it to?"

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Magically all of these routers have managed to coordinate their routing tables,

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so that when a packet gets sent here,

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it eventually makes its way to the destination.

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It's a rather remarkable system that there are now

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trillions of these packets getting routed around,

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and they all mostly manage to get to their destination,

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occasionally things go wrong and packets don't make it.

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Here's the first few lines of a trace route that I did.

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This is something that gives some diagnostics about if I'm trying to send data packets

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from my computer to si.umich.edu,

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what happens to them?

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The first line is telling me where does that packet go first.

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Well, first it goes to a computer that has the address

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100.68.0.5 and it took 71 milliseconds to get it there.

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In fact, when I run this trace route program it does

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this three times and so one time took 71 milliseconds,

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the next time it took somewhat less,

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the third time somewhere in the middle of 58.4 milliseconds.

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The second line is saying where does

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a data packet go if it's eventually trying to get to si.umich.edu,

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where does it go next?

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Well, it went from a computer at address 100.68.0.5

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to a computer at 172.19.242.241.

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We can see this is called two hops;

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the first hop went to 100.68,

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the second hop to 172.19,

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the third hop was another router at 10.255.255.253, and so on.

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I haven't shown you all the way,

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but after eight or 10 hops,

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the packets eventually get to si.umich.edu.

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Typically to go two hops it takes a little longer than to go one hop.

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So, that gives you a little sense of what's going on behind the scenes.

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We're not going to have to deal with all of this technical detail.

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I just wanted to give you a sense of what's happening

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when one computer is talking to another.

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So, that's the basics of Internet routing,

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how one computer can send data to another computer by

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way of intermediate routers. We'll see you next time.