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[music]

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So we have spent the vast

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majority of our day today

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talking about taking networks,

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a single network that you've

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leased or rented from your

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service provider,

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and by modifying some of the

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host bits,

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taking that network and cutting

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it up into little subnets,

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cutting it up into

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multiple pieces,

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replaced on this LAN over here,

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that LAN over here,

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this LAN over here.

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Summarization is just the

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opposite of that - starting out

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with a bunch of networks

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and saying,

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I want to shrink them all down

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to something they have

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in common.

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Summarization is the process

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of that.

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So it's the process of

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combining multiple subnetworks

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into a single

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network advertisement.

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Also, we'll refer to this as

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route aggregation.

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Efficient and large networks

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provides addressing hierarchy.

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Okay, so let's talk a little

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bit about why would we want to

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do this.

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I'm going to use my

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favorite analogy.

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Ready? Here we go.

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If you've seen any videos

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of me,

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you'll see me do this a lot.

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I'm a router and you're

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a router,

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and we're connected together.

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I do that all the time so play

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along with me here.

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I'm a router and you're

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a router.

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We have a cable between us,

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and we're running some kind of

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a routing protocol.

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Take your pick: RIP, OSPF,

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EIGRP, BGP.

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It doesn't matter.

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The fact is,

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the networks I know about,

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it's my responsibility to

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advertise them to you so that

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you know you can get to them

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through me.

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That's the whole point.

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Now normally when you turn on a

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routing protocol - like I said,

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take your pick - as that router

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learns about networks,

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it doesn't matter if they're

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subnets or classful networks.

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The router doesn't care.

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The router says,

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Hey, let met tell you,

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neighbor, about what I know.

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So imagine if behind me there's

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a whole bunch of LANs.

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Let's say there's a LAN out

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there that's 10.10.0.0/16,

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10.30.0.0/16,

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10.64.128/17.

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Now normally,

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as a routing protocol,

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I would send all those to you.

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I'd say, Okay,

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here's your routing update.

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Let me tell you about 10.10/16,

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10.30/16,

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10.40.120.

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So I'd give you all these

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specific networks that

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I know about.

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Well, there can be a downside

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to that.

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What if you are running out

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of memory?

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Because all these routes that

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I'm sending to you,

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you have to store them in

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memory somewhere in a place

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called an IP routing table,

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an IPv4 routing table.

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What if you've got so many

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routes already that if I send

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you, let's say, oh, 500 routes,

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that's going to overwhelm you.

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You're going to say,

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Look, I'm busting at the

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seams here.

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I don't have enough memory left

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over to store 500

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additional routes.

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That's going to be bad.

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It might cause your router

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to crash.

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Least case scenario,

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you're probably going to drop

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some of my routes because

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you're just not going to have

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any place to put them.

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And certainly,

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when we're talking about

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service provider networks,

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this is a big concern.

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If we're talking about a router

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that's owned by Time Warner

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Cable or Comcast,

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that router could literally,

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if it knows about everything in

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the Internet--

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currently in the Internet

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there's well over

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400,000 networks.

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Think about that,

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400,000 networks in the

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entire world.

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Imagine how much memory that

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would consume to store in a

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single routing table.

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And that's not the only thing

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that routers keep in track of.

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That router is also keeping

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track of access lists and ARP

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tables and all kinds of

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other stuff.

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And now he's got to store

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500,000 routes?

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Yikes. Better be a big,

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powerful router with a lot of

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memory to do that.

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Well, a lot of times,

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what service providers will do

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is they'll say,

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look, yeah,

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I could send you all these 500

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routes I know about: 10.10,

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10.30, 10.90.128.

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But when I think about it,

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they all begin with 10.

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Every router I have back here

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begins with 10,

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so why don't I do this?

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Why don't I just send you a

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single route that says,

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hey, look, any packet you have

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that starts with 10,

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10/8, send it to me.

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I know about all networks that

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begin with 10.

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That'd be nice wouldn't it,

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because that, now,

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conserves your memory.

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You don't have to know about

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every specific little thing.

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I'm just sending you a single

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aggregated route,

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a single summarized route.

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And that's what route

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summarization is all about.

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Now, routers don't do this,

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technically, by default.

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You have to get in them and

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configure them.

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And this is something at the

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CCNP level,

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you learn how to do that.

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But theoretically on paper,

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you do have to know how to take

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a bunch of subnets like 10/1,

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10/2, 10/50, 10/98,

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and ask yourself, okay,

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if I thought about these as

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binary patterns,

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from left to right,

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is any of the binary pattern

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consistent and the same across

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all of these?

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That's what I'll send to my

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neighbor.

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So for example--

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let's look at an example

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of this.

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Here we have two networks,

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10.10.32.0 and 10.10.48.0.

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They both, in this case,

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have the exact same mask,

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although really,

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the subnet mask of the original

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network is irrelevant in the

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case of route summarization.

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We really could care less.

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So that's to serve like

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a distractor.

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So if we convert these networks

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into binary, remember,

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what is our objective here?

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What's our goal?

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We're trying to figure out,

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from left to right,

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what do these networks have in

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common, if anything.

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Well, clearly,

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they have the first 16 bits in

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common because they both start

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with 10.10.

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But do they have any more

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than that?

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When we go into the

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third octet,

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we see, Oh, wait a second,

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the number 32 and 48,

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when I start converting that

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into binary, sure enough,

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those two numbers have the

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first 3 bits in common.

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They both start with 001.

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The 128 and 64-bit are off,

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the 32-bit is on.

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After that,

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I start experiencing some

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variation where they're

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not the same.

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So what do these guys have

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in common?

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What they have in common is the

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first 19 bits.

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All 8 bits of 10,

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all 8 bits of 10 in the second

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octet and then 3 bits of 001.

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So I can shrink that down to a

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single summarized route of

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10.10.32.0/19.

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And that's how you summarize

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something like that.

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Well, let's do one--

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let's see here.

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Actually, I've got a whiteboard

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pre-built that we can work on

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this. There we go.

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Let's do a couple examples

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of this.

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Let's start with these three

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networks right here.

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So we can see pretty clearly

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right off the bat that all

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three of these networks start

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with 20.20.40,

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so their first 24 bits,

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clearly, are in common.

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Well let's see if we can create

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summarized route that's even

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more specific than that,

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that has even more in common.

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So, in the fourth octet, .90,

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let's see here.

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It's got the 128-bit

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turned off.

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Also, 82 has the 128-bit

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turned off,

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and 88 has the 128-bit

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turned off.

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So far, so good.

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Let's keep going.

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.90 has the 64-bit turned on,

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82 has the 64-bit turned on,

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and 88 has the 64-bit

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turned on.

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So far we've identified 26 bits

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that all three of these

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networks have in common.

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Can we go further?

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Can we keep going?

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Let's see.

291
00:08:43,060 --> 00:08:46,189
The 32-bit is off here because

292
00:08:46,190 --> 00:08:48,446
64 plus 32 would be 96.

293
00:08:48,447 --> 00:08:50,658
So that's off.

294
00:08:50,683 --> 00:08:53,220
32-bit is off here,

295
00:08:53,221 --> 00:08:54,941
32-bit is off here.

296
00:08:54,941 --> 00:08:55,831
Oh, so far, so good.

297
00:08:55,832 --> 00:08:56,404
Let's keep going.

298
00:08:56,405 --> 00:08:58,560
Can we go even further?

299
00:08:58,560 --> 00:09:00,381
What about the 16-bit?

300
00:09:00,382 --> 00:09:05,354
64 plus 16 is 80.

301
00:09:05,379 --> 00:09:07,388
So yeah, 16-bit is on,

302
00:09:07,389 --> 00:09:08,885
on and on.

303
00:09:08,910 --> 00:09:11,557
So we're up to 80 right now.

304
00:09:11,582 --> 00:09:13,785
Now if I go to the 8-bit,

305
00:09:13,810 --> 00:09:15,329
80 plus 8 is 88.

306
00:09:15,330 --> 00:09:17,922
Yeah, we got the 8-bit on here.

307
00:09:17,922 --> 00:09:21,729
The 8-bit is off there and the

308
00:09:21,730 --> 00:09:23,831
8-bit is on right here.

309
00:09:23,831 --> 00:09:25,836
So now I can stop.

310
00:09:25,861 --> 00:09:27,589
I can see that of

311
00:09:27,590 --> 00:09:29,646
these networks,

312
00:09:29,671 --> 00:09:30,647
in addition to the first

313
00:09:30,648 --> 00:09:32,833
24-bits in common,

314
00:09:33,762 --> 00:09:34,932
they actually have the first

315
00:09:34,932 --> 00:09:38,506
28-bits in common.

316
00:09:40,745 --> 00:09:42,204
So all these networks start

317
00:09:42,205 --> 00:09:49,221
with 20.20.40.0101 and then I

318
00:09:49,221 --> 00:09:52,490
have my variability.

319
00:09:52,491 --> 00:09:53,687
So that would come up with

320
00:09:53,688 --> 00:10:00,380
20.20.40.0101000.

321
00:10:00,405 --> 00:10:03,007
So my single summarized route

322
00:10:05,302 --> 00:10:09,940
is 20.20.40 - just make sure I

323
00:10:09,940 --> 00:10:11,362
got this right here - 64 and

324
00:10:11,363 --> 00:10:16,766
16, yes, 80/28.

325
00:10:21,098 --> 00:10:22,976
So all 3 of these subnets are

326
00:10:22,977 --> 00:10:25,453
encompassed or included in this

327
00:10:25,454 --> 00:10:27,558
one aggregated or

328
00:10:27,583 --> 00:10:29,756
summarized route.

329
00:10:33,548 --> 00:10:34,873
Let's do one more example of

330
00:10:34,873 --> 00:10:39,785
that, down here.

331
00:10:43,364 --> 00:10:46,968
So these clearly have the first

332
00:10:46,969 --> 00:10:50,481
two bytes in common,

333
00:10:51,981 --> 00:10:53,038
so our variability is going to

334
00:10:53,039 --> 00:10:53,988
be somewhere in the

335
00:10:53,989 --> 00:10:55,675
third octet.

336
00:10:59,159 --> 00:11:00,646
They all have the 128

337
00:11:00,647 --> 00:11:02,263
turned off.

338
00:11:04,017 --> 00:11:05,200
They all have the 64

339
00:11:05,200 --> 00:11:06,606
turned off.

340
00:11:11,108 --> 00:11:12,556
They all have the 32

341
00:11:12,581 --> 00:11:14,008
turned off.

342
00:11:17,096 --> 00:11:17,957
They all have the 16

343
00:11:17,958 --> 00:11:19,931
turned off.

344
00:11:22,348 --> 00:11:24,485
This has the 8 on.

345
00:11:24,486 --> 00:11:26,720
This has the 8-bit on.

346
00:11:26,721 --> 00:11:30,280
This has the 8-bit on.

347
00:11:30,280 --> 00:11:31,915
This has the 4-bit on.

348
00:11:31,916 --> 00:11:34,347
8 plus 4 is 12.

349
00:11:34,348 --> 00:11:36,065
This has the 4 bit on.

350
00:11:36,066 --> 00:11:37,204
This does not.

351
00:11:37,205 --> 00:11:39,674
This has the 4-bit turned off.

352
00:11:39,699 --> 00:11:41,618
So now I can stop.

353
00:11:42,944 --> 00:11:44,452
In this particular case,

354
00:11:46,356 --> 00:11:49,446
they all have this bit pattern

355
00:11:49,471 --> 00:12:01,854
in common, 130.75.00001.

356
00:12:01,854 --> 00:12:08,129
Let's see 1, 2, 3, 4, 5, 6 7, 8.

357
00:12:08,129 --> 00:12:09,448
So if I convert that down,

358
00:12:09,449 --> 00:12:18,496
that's 130.75.--

359
00:12:18,496 --> 00:12:33,136
which is 130.75.8.0/16 plus 5, 21

360
00:12:33,136 --> 00:12:34,410
So there is my single

361
00:12:34,411 --> 00:12:37,977
summarized route for

362
00:12:37,977 --> 00:12:40,343
these three.

363
00:12:40,343 --> 00:12:41,928
So I've got one quiz question

364
00:12:41,929 --> 00:12:44,220
here to give you, guys

365
00:12:44,221 --> 00:12:46,681
practice on this yourself.

366
00:12:46,706 --> 00:12:47,725
There you go.

367
00:12:47,726 --> 00:12:48,636
So do the same thing.

368
00:12:48,637 --> 00:12:49,837
See if you can come up with the

369
00:12:49,838 --> 00:12:52,945
most specific summarized route

370
00:12:53,802 --> 00:12:55,458
that will collapse all three of

371
00:12:55,459 --> 00:12:57,652
those subnets down into one

372
00:12:57,677 --> 00:12:59,053
summarized route.

373
00:12:59,053 --> 00:13:00,227
I'll give you 30 seconds

374
00:13:00,228 --> 00:13:00,895
for this.

375
00:13:07,563 --> 00:13:09,460
Okay. Time's up.

376
00:13:09,461 --> 00:13:11,148
Here is our answer.

377
00:13:12,789 --> 00:13:15,352
So all three of these networks

378
00:13:16,850 --> 00:13:20,064
had the first 27 bits in

379
00:13:20,064 --> 00:13:23,326
common, 135.75.42,

380
00:13:23,326 --> 00:13:25,099
and then on the fourth octet,

381
00:13:25,124 --> 00:13:28,446
they all had 011 in common,

382
00:13:30,350 --> 00:13:37,742
which gives us 135.75.42.96

383
00:13:37,767 --> 00:13:40,054
with a /27 subnet mask.

384
00:13:41,328 --> 00:13:41,771
All right.

385
00:13:41,771 --> 00:13:44,465
We have one topic left to go,

386
00:13:44,465 --> 00:13:45,247
which we'll actually get

387
00:13:45,248 --> 00:13:46,246
through pretty quickly.

388
00:13:46,246 --> 00:13:48,084
One last topic

389
00:13:50,377 --> 00:13:52,686
that's called supernetting.

390
00:13:55,480 --> 00:13:59,642
So, supernetting is essentially

391
00:13:59,643 --> 00:14:03,378
exactly the same thing as route

392
00:14:03,378 --> 00:14:04,980
summarization that we just

393
00:14:04,981 --> 00:14:05,962
looked at.

394
00:14:05,963 --> 00:14:07,487
The logic that you used to do

395
00:14:07,488 --> 00:14:09,284
supernetting is exactly the

396
00:14:09,285 --> 00:14:10,546
same logic you use to

397
00:14:10,547 --> 00:14:12,319
do summarization.

398
00:14:12,320 --> 00:14:13,022
There's only one

399
00:14:13,023 --> 00:14:15,837
main differentiator.

400
00:14:15,838 --> 00:14:16,785
With summarization,

401
00:14:16,785 --> 00:14:17,419
look at that last

402
00:14:17,420 --> 00:14:18,760
bullet point there.

403
00:14:18,761 --> 00:14:19,768
Whatever the summarized

404
00:14:19,768 --> 00:14:20,903
route is,

405
00:14:20,904 --> 00:14:22,694
it does not break the

406
00:14:22,695 --> 00:14:24,985
classful boundaries.

407
00:14:24,986 --> 00:14:25,824
What do I mean?

408
00:14:25,825 --> 00:14:26,279
Let's go back to

409
00:14:26,279 --> 00:14:31,024
this here, 135.

410
00:14:31,049 --> 00:14:32,165
Technically, if we were talking

411
00:14:32,166 --> 00:14:33,908
about classful addresses,

412
00:14:33,933 --> 00:14:35,777
what class of address would 135

413
00:14:35,778 --> 00:14:37,737
fall into?

414
00:14:39,808 --> 00:14:42,461
Hopefully you said Class B.

415
00:14:42,462 --> 00:14:44,725
135 is Class B.

416
00:14:44,750 --> 00:14:45,688
Which means that if I'm

417
00:14:45,688 --> 00:14:47,762
doing summarization,

418
00:14:47,787 --> 00:14:49,283
whatever subnet mask

419
00:14:49,283 --> 00:14:49,930
I come up with,

420
00:14:49,931 --> 00:14:52,470
one of my summarized route,

421
00:14:52,495 --> 00:14:56,092
it has to be /16 or bigger.

422
00:14:56,093 --> 00:14:56,760
Like in this case,

423
00:14:56,760 --> 00:15:00,228
it would be /27, /16 or bigger.

424
00:15:01,498 --> 00:15:04,888
Well, supernetting is coming up

425
00:15:04,889 --> 00:15:05,290
with the mask.

426
00:15:05,290 --> 00:15:07,807
It's actually smaller than your

427
00:15:07,808 --> 00:15:09,961
classful address.

428
00:15:09,962 --> 00:15:11,717
For example,

429
00:15:13,760 --> 00:15:15,297
take a look at this.

430
00:15:16,638 --> 00:15:18,302
Let's say that my router knew

431
00:15:18,303 --> 00:15:21,212
about these two Class C

432
00:15:21,213 --> 00:15:25,590
networks - 192.168.1 and

433
00:15:25,591 --> 00:15:29,324
192.168.2

434
00:15:29,325 --> 00:15:30,530
There's no way I could

435
00:15:30,531 --> 00:15:31,464
summarize this.

436
00:15:31,464 --> 00:15:33,301
If I want to shrink this down

437
00:15:33,301 --> 00:15:35,307
into just one advertisement and

438
00:15:35,308 --> 00:15:36,846
send it as a summarized route,

439
00:15:36,847 --> 00:15:37,973
I couldn't.

440
00:15:37,998 --> 00:15:39,314
Why not?

441
00:15:39,339 --> 00:15:41,345
Well, because the mask is

442
00:15:41,346 --> 00:15:43,334
already at the classful

443
00:15:43,335 --> 00:15:46,032
boundary of /24.

444
00:15:46,032 --> 00:15:47,422
Summarization means I can't go

445
00:15:47,423 --> 00:15:49,282
any less than a /24 if what I'm

446
00:15:49,283 --> 00:15:51,364
starting with is essentially a

447
00:15:51,365 --> 00:15:53,075
Class C.

448
00:15:53,100 --> 00:15:54,451
This is where supernetting

449
00:15:54,452 --> 00:15:55,393
comes into play.

450
00:15:55,394 --> 00:15:56,859
So use the exact same logic.

451
00:15:56,860 --> 00:15:57,757
Let's look at this and say for

452
00:15:57,757 --> 00:15:58,873
a second, Okay, well,

453
00:15:58,898 --> 00:15:59,680
from left to right,

454
00:15:59,681 --> 00:16:00,879
if I was to convert this into

455
00:16:00,880 --> 00:16:03,557
binary, from left to right,

456
00:16:03,582 --> 00:16:04,977
what do these two networks have

457
00:16:04,978 --> 00:16:06,172
in common?

458
00:16:07,830 --> 00:16:09,011
The first two octets

459
00:16:09,012 --> 00:16:10,658
were the same.

460
00:16:10,683 --> 00:16:13,807
And then in the third octet--

461
00:16:17,085 --> 00:16:18,724
in the third octet,

462
00:16:18,749 --> 00:16:24,863
they're the same up to here.

463
00:16:24,864 --> 00:16:26,360
So really this first--

464
00:16:26,361 --> 00:16:27,960
these two networks both have in

465
00:16:27,961 --> 00:16:31,671
common the first 22-bits.

466
00:16:33,600 --> 00:16:35,122
So I could collapse them down

467
00:16:35,123 --> 00:16:36,802
into one route,

468
00:16:36,803 --> 00:16:42,030
192.168.0.0/22 and this is

469
00:16:42,055 --> 00:16:44,566
called a supernet.

470
00:16:44,566 --> 00:16:45,877
Because even though if you take

471
00:16:45,878 --> 00:16:47,991
a look at that first byte,

472
00:16:48,016 --> 00:16:49,942
you'd say, Oh 192.

473
00:16:49,943 --> 00:16:52,094
Well in binary that's 110,

474
00:16:52,094 --> 00:16:54,255
that's Class C.

475
00:16:54,256 --> 00:16:56,407
But Class C means /24 and here

476
00:16:56,407 --> 00:16:59,715
I'm doing a /22.

477
00:16:59,715 --> 00:17:00,765
That's what makes it

478
00:17:00,766 --> 00:17:03,944
a supernet.

479
00:17:03,945 --> 00:17:04,755
That's what makes it

480
00:17:04,755 --> 00:17:06,355
a supernet.

481
00:17:06,355 --> 00:17:07,618
Now as far as summarization

482
00:17:07,619 --> 00:17:09,453
is concerned.

483
00:17:09,454 --> 00:17:12,735
Some routing protocols,

484
00:17:12,736 --> 00:17:13,961
when you turn them on,

485
00:17:13,961 --> 00:17:14,916
and it depends on the version

486
00:17:14,916 --> 00:17:15,993
of software you're dealing with

487
00:17:15,994 --> 00:17:17,087
in the code,

488
00:17:17,112 --> 00:17:18,315
some routers perform route

489
00:17:18,316 --> 00:17:20,666
summarization by default.

490
00:17:20,666 --> 00:17:22,293
They will take some subnets,

491
00:17:22,318 --> 00:17:23,589
shrink them down into a single

492
00:17:23,589 --> 00:17:25,345
route, advertise it out,

493
00:17:25,370 --> 00:17:26,480
and the subnet mask--

494
00:17:26,481 --> 00:17:27,833
the mask, I should say.

495
00:17:27,834 --> 00:17:28,897
I shouldn't say subnet mask.

496
00:17:28,897 --> 00:17:30,846
The mask associated with that

497
00:17:30,847 --> 00:17:34,027
summarized route does not break

498
00:17:34,028 --> 00:17:34,637
the rules of

499
00:17:34,638 --> 00:17:36,891
classful addressing.

500
00:17:38,132 --> 00:17:39,621
Supernetting can only be

501
00:17:39,622 --> 00:17:41,485
done manually.

502
00:17:41,486 --> 00:17:42,546
Typically this is something you

503
00:17:42,547 --> 00:17:44,487
would see done in BGP.

504
00:17:44,512 --> 00:17:45,787
You wouldn't normally see it

505
00:17:45,788 --> 00:17:47,422
done in your interior gateway

506
00:17:47,422 --> 00:17:49,594
protocols, although you can.

507
00:17:49,595 --> 00:17:50,635
But this is usually something

508
00:17:50,635 --> 00:17:51,848
that ISPs would do to

509
00:17:51,849 --> 00:17:52,742
each other.

510
00:17:52,743 --> 00:17:53,689
For example,

511
00:17:53,689 --> 00:17:56,350
if I'm Time Warner Cable,

512
00:17:56,375 --> 00:17:57,761
I might say, well--

513
00:17:57,786 --> 00:18:00,021
let's do an example of that.

514
00:18:00,046 --> 00:18:01,138
Let's say that we have here

515
00:18:01,139 --> 00:18:02,872
two routers.

516
00:18:05,045 --> 00:18:06,750
This router here is owned by

517
00:18:06,750 --> 00:18:08,062
Time Warner Cable.

518
00:18:08,087 --> 00:18:09,608
This router here is owned

519
00:18:09,609 --> 00:18:12,177
by Sprint.

520
00:18:12,178 --> 00:18:14,427
Two different routing domains.

521
00:18:14,428 --> 00:18:15,612
Now, Time Warner Cable,

522
00:18:15,613 --> 00:18:17,462
as a service provider,

523
00:18:17,487 --> 00:18:19,378
own thousands and thousands,

524
00:18:19,379 --> 00:18:21,124
tens, probably even hundreds of

525
00:18:21,125 --> 00:18:23,095
thousands of subnets.

526
00:18:23,096 --> 00:18:23,686
Let's say they say,

527
00:18:23,687 --> 00:18:24,550
okay, well,

528
00:18:24,551 --> 00:18:26,116
I own every subnet from

529
00:18:26,117 --> 00:18:28,426
1.anything,

530
00:18:29,751 --> 00:18:34,709
2.anything, 3.anything.

531
00:18:34,710 --> 00:18:35,951
Let's see if I can figure out a

532
00:18:35,952 --> 00:18:40,718
nice, easy way to do this.

533
00:18:43,303 --> 00:18:45,025
Let's do Class B instead of

534
00:18:45,025 --> 00:18:46,873
Class A.

535
00:18:46,898 --> 00:18:55,501
Let's say 130.1.0.0, 130.2.0.0,

536
00:18:57,068 --> 00:19:01,871
130.3.0.0.

537
00:19:05,196 --> 00:19:06,419
Let's stick up here at the top

538
00:19:06,420 --> 00:19:08,917
130.0.0.0

539
00:19:10,661 --> 00:19:12,054
So Time Warner Cable actually

540
00:19:12,055 --> 00:19:12,721
owns the--

541
00:19:12,722 --> 00:19:13,854
I'm just making this up here.

542
00:19:13,855 --> 00:19:15,091
Time Warner Cable,

543
00:19:15,092 --> 00:19:15,673
let's assume,

544
00:19:15,673 --> 00:19:19,906
owns these four full Class Bs.

545
00:19:19,931 --> 00:19:20,629
Now, chances are,

546
00:19:20,630 --> 00:19:21,517
they're probably not going to

547
00:19:21,518 --> 00:19:22,635
hand out these Class Bs to

548
00:19:22,636 --> 00:19:24,686
their customers because each

549
00:19:24,686 --> 00:19:27,588
one of those Class Bs supports

550
00:19:27,588 --> 00:19:30,248
over 65,000 hosts.

551
00:19:30,249 --> 00:19:31,955
So if a customer goes to Time

552
00:19:31,956 --> 00:19:32,924
Warner Cable and they say,

553
00:19:32,925 --> 00:19:34,655
I need a network from you.

554
00:19:34,680 --> 00:19:35,797
Time Warner Cable might say,

555
00:19:35,798 --> 00:19:37,148
okay, well let's see here.

556
00:19:37,173 --> 00:19:39,271
How many hosts does your

557
00:19:39,271 --> 00:19:40,573
network support?

558
00:19:40,574 --> 00:19:42,119
And if the customer says,

559
00:19:42,119 --> 00:19:45,446
my network supports 300 hosts,

560
00:19:45,471 --> 00:19:46,679
Time Warner Cable is not going

561
00:19:46,680 --> 00:19:47,431
to give them this entire

562
00:19:47,432 --> 00:19:48,445
Class B.

563
00:19:48,470 --> 00:19:49,984
They'll probably carve out a

564
00:19:49,985 --> 00:19:52,482
subnet of that Class B and sell

565
00:19:52,483 --> 00:19:54,044
that to the customer.

566
00:19:54,044 --> 00:19:55,127
But Time Warner Cable,

567
00:19:55,128 --> 00:19:56,180
as a whole, as a company,

568
00:19:56,180 --> 00:19:58,510
is responsible for these four

569
00:19:58,510 --> 00:19:59,075
Class Bs.

570
00:19:59,076 --> 00:20:00,869
They've been allocated these

571
00:20:00,894 --> 00:20:01,461
from the Regional

572
00:20:01,462 --> 00:20:03,730
Internet Registry.

573
00:20:03,755 --> 00:20:08,353
Well, using BGP or something,

574
00:20:08,378 --> 00:20:11,210
They could send all four of

575
00:20:11,211 --> 00:20:14,430
these to Sprint,

576
00:20:14,455 --> 00:20:15,253
or they could say,

577
00:20:15,254 --> 00:20:16,793
Let's do supernetting here.

578
00:20:16,793 --> 00:20:17,656
What do these guys have

579
00:20:17,656 --> 00:20:18,397
in common?

580
00:20:18,398 --> 00:20:19,227
Well, they all start out

581
00:20:19,227 --> 00:20:21,861
with 130,

582
00:20:21,862 --> 00:20:24,410
and then in the second octet,

583
00:20:24,435 --> 00:20:32,210
128, 64, 32, 16, 8, and 4,

584
00:20:32,210 --> 00:20:34,518
those bits are all zeroed out,

585
00:20:36,153 --> 00:20:37,256
and the last two bits have

586
00:20:37,257 --> 00:20:39,027
various combinations.

587
00:20:44,542 --> 00:20:45,573
So Time Warner Cable could

588
00:20:45,574 --> 00:20:47,372
instead send a supernet,

589
00:20:47,372 --> 00:20:48,423
which looks like this:

590
00:20:48,423 --> 00:20:55,736
130.0.0.0/14.

591
00:20:55,737 --> 00:20:56,956
All four of these networks have

592
00:20:56,957 --> 00:20:59,692
the first 14 bits in common -

593
00:20:59,693 --> 00:21:02,389
130 and then 6 bits of 0.

594
00:21:02,390 --> 00:21:03,793
This is a supernet.

595
00:21:09,761 --> 00:21:10,514
So when performing

596
00:21:10,515 --> 00:21:12,357
summarization or supernetting,

597
00:21:12,358 --> 00:21:12,968
like I said,

598
00:21:12,969 --> 00:21:14,489
the logic you work through is

599
00:21:14,490 --> 00:21:15,998
exactly the same.

600
00:21:16,023 --> 00:21:17,711
Simply ask yourself,

601
00:21:17,736 --> 00:21:20,960
what bits from left to right do

602
00:21:20,961 --> 00:21:22,306
all of these networks have

603
00:21:22,307 --> 00:21:23,293
in common?

604
00:21:23,318 --> 00:21:27,992
[music]