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[music]

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All this time with subnetting,

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we've been doing something

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called same length subnetting and

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there is a problem to

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this approach.

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The problem is we've been

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designing these networks purely

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from the perspective of how

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many networks do I need?

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Creating a subnet mask that's

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the same for every single

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network based on how many total

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quantity of networks I need.

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It was not even

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a consideration.

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We didn't even think about how

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many hosts do we need in each

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one of those networks.

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That would result in a problem.

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For example,

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take a look at this.

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We start here with a single

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base network a /24.

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That one network gives us the

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whole range of host space on

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that last octet we've got from

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0 to .255 that's the full

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range of host addresses.

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Now we know that 2 of those we

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can't use right the .0

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and the .255,

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but that circle there

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represents the full range of

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host space in that last octet.

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Now if we used our previous

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approach of same length

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subnetting, and let's say

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we needed 4 networks

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like this building

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right here needs 4 networks,

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well what's the approach we

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would have come up with?

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Well we would have said,

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Okay, if I steal 2 networking

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bits, if I steal 2 subnet bits,

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there I go.

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That will give me

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4 subnets right?

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So now we got that

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circle there.

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We divided it into 4 sections.

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A pie of 4 equal slices,

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and they're all equal because

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every single network is going

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to have the same subnet mask.

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They're all going to be /26,

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and with a /26,

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what do we know?

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Going back to this right here

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with the /26,

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we know that each one of these

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networks is going to have 62

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host addresses.

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So 64 addresses total - the

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network address,

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62 host addresses,

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and then the broadcast address.

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So 64 addresses total.

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That's what you see right here,

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network 1, 2, 3, and 4,

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and they're all equal.

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Network 1 starts at the network

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address of .0,

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broadcast address ends on .63.

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Network 2 begins at address 64,

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ends at broadcast address .127.

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Network 3 begins at network

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address 128,

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broadcast address .191,

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and then the remaining host

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space is network number 4.

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So we do have our

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4 networks,

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but we have a problem here.

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Check it out.

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Network number 2,

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it requires more than 62 hosts.

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He needs 80 hosts.

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80 hosts won't fit into any of

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these slices of pie right here.

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It's not going to fit.

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Not only that,

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look at some of these

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other networks.

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For example,

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look at network number 1.

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There's only 5 hosts

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in that,

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and yet we've allocated 62

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hosts in that one network.

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That's far more host space than

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we really need.

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We've got a whole bunch of

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extra bits that aren't even

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being used because this network

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is far larger and can support

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far more hosts than is

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actually needed.

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So this is the problem with

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same-length subnetting.

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We're completely ignoring our

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host requirements,

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which means we might end up

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with subnets that are either

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far larger than we actually

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need and it's a complete waste

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of space,

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or just the opposite.

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We might come up with subnets

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that are too small like these.

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None of these subnets here will

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fit our requirement for network

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number 2.

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So this is where we take a

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different approach,

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something called

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variable-length subnet masking.

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Now with variable-length

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subnet masking,

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here's basically what we're

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going to do.

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Instead of starting with the

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position of,

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Okay, I need to figure out how

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many total networks I need,

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and then create one subnet mask

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that will come up with that

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total quantity of networks.

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We're going to do it from a

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different approach.

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Instead we're going to say,

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I'm going to create my subnets

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based on how many hosts are

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needed in each subnet.

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Oh, this subnet's really large.

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It needs a lot of hosts.

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Okay, that'll have a subnet

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mask of maybe a /26.

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Oh, and then there's a tiny

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little subnet over here that

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only needs four hosts.

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Don't need a /26 for that one.

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Why don't we use a /29

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for that one?

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And so we're going to create

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subnet masks of variable sizes

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hence the name variable length

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subnet masking.

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They're finely tuned or finely

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designed based on the host

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requirements that each network

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actually needs.

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So let's do the same

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example here.

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In this particular case,

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based on subnets,

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based on host required,

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so typically the way you do

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this is you start with the

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largest network,

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the network that's going to

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have the biggest

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host requirement.

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So I say, Okay,

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let's see here. 80 hosts.

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I need 80 hosts.

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Well going back to my chart,

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if I need 80 hosts

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what's that mean?

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That means I'm going to have to

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use a /25, doesn't it?

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Because a /25 will give me

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126 hosts.

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Anything else like a /26 or /27

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won't support the 80 hosts that

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I need.

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So for that particular slice

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of the pie,

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it's going to be pretty big

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because it's going to support

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126 hosts.

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There we go.

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That's what I'm going to divvy

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up for network number 2, a /25.

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So .0 is going to be my

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network address,

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.127 is going to be my

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broadcast address,

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and then I'll have 126 hosts in

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the middle for use.

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I'll use 80 of them and that

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gives me a little bit for

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future expansion.

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Now we go to the next one.

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We say, Okay,

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what's the next largest one?

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22 hosts, okay,

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let's go back to here again.

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22 hosts, well,

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it looks like according to this

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to get 22 hosts,

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I would need a /27.

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That'll get me up to 30 hosts.

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All right,

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so my next network is going to

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be a /27.

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Not quite as large of a pie

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I should say the piece of the pie

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is not as large as the

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other one.

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With /27 so notice here.

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In my first network,

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I already used up all the

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addresses from .0 to .127.

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Those were used up in the

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first subnet.

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I know that the next subnet,

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regardless of how big it's

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going to be,

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even before I look at the

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next subnet,

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what do I know about it.

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I know it's going to start with

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.128 because that's the next

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available address.

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The previous one ended on .127

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so the next one is going to

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start on .128.

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I already know my starting

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network number is going to be

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199.199.199.128.

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The only thing I need to figure

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out is,

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what's the subnet mask going to

246
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be for that?

247
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Well, I just did that.

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I said, Well,

249
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based on 22 hosts the subnet

250
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mask needs to be a /27.

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With the /27 we saw that gives

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me a total of 32 addresses.

253
00:08:02,048 --> 00:08:04,498
30 host addresses plus the

254
00:08:04,499 --> 00:08:06,508
network plus the broadcast

255
00:08:06,509 --> 00:08:10,045
gives me 32 addresses.

256
00:08:10,045 --> 00:08:11,186
So I take my starting network

257
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of 128 and say,

258
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Okay, let's see.

259
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What's 128 plus 32?

260
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Oh, that's 160,

261
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so that's my next network.

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So I know now that network

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number 3 is going to be the

264
00:08:21,704 --> 00:08:22,711
128 network.

265
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His broadcast address

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will be 159,

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and then the next network

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available will be .160.

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I'll say, Okay, let's see here.

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The next largest one is network

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number 4,

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so apparently network number 4

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is going to be 199.199.199.160.

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That's where it's going

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to start.

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So we rinse and repeat.

277
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So the question now is,

278
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Okay, how big of a slice of the

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pie does that network need?

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What kind of subnet mask do I

281
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need for that?

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Well, 12 hosts.

283
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Let's see here.

284
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If I go back,

285
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12 hosts that means I

286
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need a /28.

287
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/28 will give me 14 hosts.

288
00:09:10,389 --> 00:09:13,122
So with a /28 I have a total of

289
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16 host combinations.

290
00:09:16,554 --> 00:09:17,046
Right I got the

291
00:09:17,047 --> 00:09:18,251
network address,

292
00:09:18,252 --> 00:09:19,881
14 hosts available,

293
00:09:19,881 --> 00:09:21,366
and the broadcast address which

294
00:09:21,367 --> 00:09:23,287
gives me 16.

295
00:09:23,288 --> 00:09:24,421
My networks are in increments

296
00:09:24,422 --> 00:09:27,931
of 16.

297
00:09:27,932 --> 00:09:29,928
So if I take 160 plus 16 that

298
00:09:29,929 --> 00:09:32,615
gives me 176.

299
00:09:32,615 --> 00:09:34,743
That will be my next and in

300
00:09:34,744 --> 00:09:36,885
this case final network.

301
00:09:38,426 --> 00:09:39,720
So my last network which is--

302
00:09:39,721 --> 00:09:41,105
notice what I was doing here.

303
00:09:41,106 --> 00:09:42,269
I started with the

304
00:09:42,270 --> 00:09:43,271
largest network,

305
00:09:43,272 --> 00:09:45,623
figured out the appropriate

306
00:09:45,624 --> 00:09:47,656
subnet mask for that network,

307
00:09:49,680 --> 00:09:51,094
figured out where that network

308
00:09:51,095 --> 00:09:53,406
ended, and then I said,

309
00:09:53,407 --> 00:09:54,823
Okay, now I know the next

310
00:09:54,824 --> 00:09:56,461
available address will start my

311
00:09:56,462 --> 00:09:58,698
next network, and then I

312
00:09:58,699 --> 00:09:59,431
repeated the process,

313
00:09:59,432 --> 00:10:01,365
and I went from the largest all

314
00:10:01,366 --> 00:10:02,665
the way down to smallest so in

315
00:10:02,666 --> 00:10:04,326
this particular case Network 1

316
00:10:04,327 --> 00:10:06,194
is the smallest.

317
00:10:07,399 --> 00:10:09,109
That I could get by with a

318
00:10:09,110 --> 00:10:11,816
/29 mask.

319
00:10:11,816 --> 00:10:13,880
/29's are always in multiples

320
00:10:13,881 --> 00:10:15,636
or increments of 8's.

321
00:10:18,076 --> 00:10:18,317
All right,

322
00:10:18,318 --> 00:10:22,799
if we go back to this /29's

323
00:10:22,800 --> 00:10:25,090
always in increments of 8's.

324
00:10:28,800 --> 00:10:31,499
If it started at .176,

325
00:10:31,500 --> 00:10:36,223
176 plus 8 is .184 so that's

326
00:10:36,224 --> 00:10:37,165
where that little slice is

327
00:10:37,166 --> 00:10:38,602
going to fit for Network 1 and

328
00:10:38,603 --> 00:10:40,263
now I've got all this space

329
00:10:40,264 --> 00:10:44,709
here from .184 to .255 as all

330
00:10:44,710 --> 00:10:45,748
unused space for

331
00:10:45,749 --> 00:10:47,471
future expansion.

332
00:10:48,918 --> 00:10:50,036
I could have more networks in

333
00:10:50,037 --> 00:10:51,227
the future if my

334
00:10:51,228 --> 00:10:52,850
building grows.

335
00:10:52,850 --> 00:10:54,262
So with same length subnet

336
00:10:54,263 --> 00:10:56,993
masking all we cared about was

337
00:10:56,994 --> 00:10:58,057
the quantity of networks that

338
00:10:58,058 --> 00:10:59,623
we needed.

339
00:10:59,624 --> 00:11:01,527
Therefore, each resulting subnet

340
00:11:01,527 --> 00:11:05,040
had the exact same subnet mask.

341
00:11:05,041 --> 00:11:07,235
But that was very risky.

342
00:11:07,236 --> 00:11:08,411
Some of those subnets might not

343
00:11:08,412 --> 00:11:10,030
be big enough for our really

344
00:11:10,031 --> 00:11:12,090
big networks.

345
00:11:12,091 --> 00:11:13,135
With variable-length

346
00:11:13,136 --> 00:11:14,718
subnet masking,

347
00:11:14,719 --> 00:11:16,180
we're creating each subnet to

348
00:11:16,181 --> 00:11:17,935
be finally tailored to the host

349
00:11:17,936 --> 00:11:19,646
requirements of that

350
00:11:19,647 --> 00:11:21,407
particular network.

351
00:11:22,645 --> 00:11:24,012
Actually, before we talk about

352
00:11:24,013 --> 00:11:26,276
Classless Inter-Domain Routing,

353
00:11:26,277 --> 00:11:27,725
let's just do one more example

354
00:11:27,726 --> 00:11:30,529
of VLSM before I throw a couple

355
00:11:30,530 --> 00:11:32,714
of quiz slides at you.

356
00:11:32,715 --> 00:11:33,750
Okay, so let's say that

357
00:11:33,751 --> 00:11:34,608
we had this.

358
00:11:36,999 --> 00:11:38,397
Your starting network

359
00:11:45,787 --> 00:11:47,587
We'll make it easy,

360
00:11:47,587 --> 00:11:49,079
something that falls on a nice,

361
00:11:49,080 --> 00:11:55,384
even bit boundary - 200.200.200.0

362
00:11:59,518 --> 00:12:01,829
Here is what you need to

363
00:12:01,830 --> 00:12:04,154
create subnets.

364
00:12:05,046 --> 00:12:06,335
Let's say we've

365
00:12:07,675 --> 00:12:09,991
got engineering.

366
00:12:09,992 --> 00:12:11,422
Let's not give names to them.

367
00:12:11,423 --> 00:12:13,636
Let's just say Network 1 which

368
00:12:13,637 --> 00:12:18,074
is going to require 70 hosts.

369
00:12:20,769 --> 00:12:24,096
Network 2 which is real tiny

370
00:12:24,097 --> 00:12:28,126
that only needs 2 hosts.

371
00:12:28,127 --> 00:12:30,183
Network 3 which is going to

372
00:12:30,184 --> 00:12:31,894
require about 16 hosts,

373
00:12:35,009 --> 00:12:38,092
and Network 4 which is going to

374
00:12:38,093 --> 00:12:41,215
require 49 hosts.

375
00:12:42,544 --> 00:12:43,997
Okay, so how do we use variable

376
00:12:43,998 --> 00:12:44,926
length Subnet masking?

377
00:12:44,926 --> 00:12:45,489
Like I said,

378
00:12:45,490 --> 00:12:47,109
we start with our largest

379
00:12:47,110 --> 00:12:48,786
network, our biggest one.

380
00:12:50,111 --> 00:12:51,295
So that will clearly be Network

381
00:12:51,296 --> 00:12:54,595
number 1,

382
00:12:54,595 --> 00:12:55,812
and using our subnet

383
00:12:55,813 --> 00:12:56,505
cheat sheet,

384
00:12:56,506 --> 00:12:57,870
what kind of subnet mask do we

385
00:12:57,871 --> 00:13:00,817
need to give us at least

386
00:13:00,818 --> 00:13:02,520
70 hosts?

387
00:13:08,734 --> 00:13:12,644
Well, we need a /25,

388
00:13:12,645 --> 00:13:14,359
for the /25 that will give us

389
00:13:14,360 --> 00:13:17,379
126 hosts.

390
00:13:17,380 --> 00:13:18,533
If we go any bigger than that,

391
00:13:18,534 --> 00:13:19,774
if we do a /26,

392
00:13:19,775 --> 00:13:21,732
it's not a big enough slice of

393
00:13:21,733 --> 00:13:22,199
the pie,

394
00:13:22,200 --> 00:13:24,822
/26 only gives us 62 hosts,

395
00:13:24,822 --> 00:13:26,379
and we need 70,

396
00:13:26,380 --> 00:13:28,449
so we got you a /25.

397
00:13:33,379 --> 00:13:35,263
Okay? So we start with

398
00:13:35,264 --> 00:13:38,740
200.200.200.0

399
00:13:38,741 --> 00:13:42,673
but now it's going to be a /25,

400
00:13:42,674 --> 00:13:44,120
and we say, Okay,

401
00:13:44,121 --> 00:13:45,390
when our networks are in

402
00:13:45,392 --> 00:13:49,765
multiples of /25s--

403
00:13:49,766 --> 00:13:50,195
I should say,

404
00:13:50,196 --> 00:13:52,353
when the subnet mask is a /25,

405
00:13:52,354 --> 00:13:53,519
let me rewrite this here,

406
00:13:54,860 --> 00:13:59,406
what increment or multiple are

407
00:13:59,407 --> 00:14:00,197
the networks?

408
00:14:02,012 --> 00:14:03,582
/25s are always

409
00:14:03,583 --> 00:14:06,480
in multiples of 128.

410
00:14:08,748 --> 00:14:10,631
So if I add 128 to this,

411
00:14:10,632 --> 00:14:11,813
that will give me my

412
00:14:11,814 --> 00:14:13,023
next network.

413
00:14:13,024 --> 00:14:13,714
So let's see what's the

414
00:14:13,715 --> 00:14:14,164
next one.

415
00:14:14,165 --> 00:14:15,722
Oh, that's 49.

416
00:14:15,723 --> 00:14:16,572
Remember, I'm going from

417
00:14:16,573 --> 00:14:17,980
largest to smallest,

418
00:14:17,981 --> 00:14:22,650
so 200.200.200.128 because I

419
00:14:22,650 --> 00:14:24,485
said that was my next available

420
00:14:24,486 --> 00:14:25,596
network address.

421
00:14:27,414 --> 00:14:30,675
Okay, but this network here

422
00:14:31,471 --> 00:14:34,480
doesn't necessarily need a /25.

423
00:14:34,481 --> 00:14:35,765
That's the subnet mask that was

424
00:14:35,766 --> 00:14:36,754
appropriate for this

425
00:14:36,755 --> 00:14:38,424
size network.

426
00:14:38,424 --> 00:14:39,837
What subnet mask do we need

427
00:14:39,838 --> 00:14:41,497
that's appropriate for

428
00:14:41,497 --> 00:14:42,840
49 hosts?

429
00:14:44,680 --> 00:14:45,870
Let's go back.

430
00:14:45,871 --> 00:14:47,667
49 hosts.

431
00:14:53,228 --> 00:14:54,245
With 49 hosts,

432
00:14:55,719 --> 00:15:00,486
we would need a /26.

433
00:15:00,487 --> 00:15:02,271
That will support up to 62

434
00:15:02,273 --> 00:15:04,004
hosts with a /26.

435
00:15:05,603 --> 00:15:06,788
So we put that down here.

436
00:15:06,789 --> 00:15:08,148
That's our new subnet mask for

437
00:15:08,149 --> 00:15:09,160
this particular slice

438
00:15:09,161 --> 00:15:10,387
of the pie.

439
00:15:11,849 --> 00:15:13,119
And now we ask ourselves: okay,

440
00:15:13,120 --> 00:15:15,384
in /26s, are networks--

441
00:15:15,385 --> 00:15:17,983
are sub networks are always in

442
00:15:17,984 --> 00:15:19,617
multiples or increments

443
00:15:19,618 --> 00:15:20,855
of what?

444
00:15:22,162 --> 00:15:23,452
They're always an increments or

445
00:15:23,453 --> 00:15:27,448
multiples of 64.

446
00:15:29,087 --> 00:15:30,236
So I take my existing network

447
00:15:30,237 --> 00:15:31,589
and I add 64 to it.

448
00:15:32,339 --> 00:15:37,907
120 plus 64 is what? 192.

449
00:15:37,907 --> 00:15:38,770
That's going to be my next

450
00:15:38,771 --> 00:15:45,290
available network - 200.200.200.192

451
00:15:45,291 --> 00:15:46,518
Once again,

452
00:15:46,518 --> 00:15:47,788
don't worry about your existing

453
00:15:47,789 --> 00:15:48,795
subnet masks.

454
00:15:48,796 --> 00:15:49,816
Now we need to come up with a

455
00:15:49,817 --> 00:15:51,294
subnet mask that's appropriate

456
00:15:51,295 --> 00:15:53,089
for this network,

457
00:15:53,090 --> 00:15:54,000
16 hosts.

458
00:15:55,519 --> 00:15:58,024
Well, to support 16 hosts,

459
00:15:59,867 --> 00:16:01,801
I need a /27,

460
00:16:03,276 --> 00:16:04,407
that will give me up to

461
00:16:04,408 --> 00:16:05,367
30 hosts.

462
00:16:05,368 --> 00:16:06,933
I could almost do a /28,

463
00:16:06,934 --> 00:16:08,825
but /28 wasn't quite

464
00:16:08,825 --> 00:16:10,281
big enough.

465
00:16:10,282 --> 00:16:12,279
It was two hosts shy so I need

466
00:16:12,280 --> 00:16:13,923
to go with a /27.

467
00:16:16,461 --> 00:16:17,829
So it's /27.

468
00:16:19,683 --> 00:16:21,788
And with /27s,

469
00:16:21,789 --> 00:16:23,230
my subnets are always in

470
00:16:23,231 --> 00:16:25,237
increments of 32.

471
00:16:25,238 --> 00:16:26,146
They're always in multiples

472
00:16:26,147 --> 00:16:27,316
of 32.

473
00:16:28,183 --> 00:16:29,770
So I take the number 32 and I

474
00:16:29,771 --> 00:16:30,567
add it to my existing

475
00:16:30,567 --> 00:16:31,465
network number.

476
00:16:31,466 --> 00:16:34,854
What's 192 plus 32?

477
00:16:36,849 --> 00:16:38,558
That is 224,

478
00:16:40,099 --> 00:16:41,854
which gives me my last network.

479
00:16:44,415 --> 00:16:45,740
And finally,

480
00:16:45,741 --> 00:16:47,335
what subnet mask is appropriate

481
00:16:47,335 --> 00:16:49,093
to support two hosts?

482
00:16:51,772 --> 00:16:55,202
That fits just perfectly with a

483
00:16:55,203 --> 00:16:56,933
/30.

484
00:16:56,933 --> 00:16:59,707
/30 supports two hosts.

485
00:17:05,523 --> 00:17:07,122
And that, my friends,

486
00:17:07,123 --> 00:17:08,638
is variable-length

487
00:17:08,639 --> 00:17:10,107
subnet masking.

488
00:17:14,733 --> 00:17:17,342
Before I show a couple more

489
00:17:17,343 --> 00:17:18,549
quiz questions to you,

490
00:17:18,550 --> 00:17:21,367
what questions do you have?

491
00:17:36,843 --> 00:17:37,063
All right,

492
00:17:37,063 --> 00:17:38,142
let's give you an opportunity

493
00:17:38,143 --> 00:17:39,285
to practice this.

494
00:17:42,789 --> 00:17:44,209
I know I'm skipping the slide

495
00:17:44,210 --> 00:17:45,878
on classless

496
00:17:45,879 --> 00:17:46,819
interdomain routing.

497
00:17:46,820 --> 00:17:48,518
I will come back to that.

498
00:17:48,519 --> 00:17:49,167
I just want to give you a

499
00:17:49,168 --> 00:17:51,229
chance right now to practice

500
00:17:51,230 --> 00:17:53,839
variable length subnet masking.

501
00:17:57,426 --> 00:17:58,517
Starting with the base network

502
00:17:58,517 --> 00:17:59,923
that you see right there,

503
00:17:59,924 --> 00:18:02,028
use VLSM to divide the single

504
00:18:02,029 --> 00:18:03,177
network into multiple

505
00:18:03,178 --> 00:18:05,103
subnetworks appropriate to fit

506
00:18:05,104 --> 00:18:07,342
the requirement shown below.

507
00:18:07,343 --> 00:18:08,073
Just like it did on

508
00:18:08,074 --> 00:18:09,077
the whiteboard.

509
00:18:10,989 --> 00:18:14,184
So I will give you one minute

510
00:18:14,185 --> 00:18:16,530
to work on this.

511
00:19:13,384 --> 00:19:14,361
Okay, let's work through

512
00:19:14,362 --> 00:19:14,883
this one.

513
00:19:18,914 --> 00:19:20,148
First thing was hopefully you

514
00:19:20,149 --> 00:19:21,770
reorder them by

515
00:19:21,771 --> 00:19:23,301
largest network.

516
00:19:23,301 --> 00:19:25,033
I should say largest host

517
00:19:25,034 --> 00:19:26,805
requirement to smallest

518
00:19:26,805 --> 00:19:28,288
host requirement.

519
00:19:28,289 --> 00:19:29,891
So from what we saw before,

520
00:19:29,892 --> 00:19:31,209
the biggest network was network

521
00:19:31,210 --> 00:19:31,938
number 3,

522
00:19:31,939 --> 00:19:34,012
marketing with 100 hosts,

523
00:19:34,012 --> 00:19:35,497
so we start with that one,

524
00:19:36,390 --> 00:19:38,561
and we needed a /25 subnet mask

525
00:19:38,562 --> 00:19:41,054
to support up to 100 hosts,

526
00:19:42,819 --> 00:19:46,159
/25 is always in increments or

527
00:19:46,160 --> 00:19:47,318
multiples of 128.

528
00:19:47,319 --> 00:19:50,220
So our next network was 128 so

529
00:19:50,221 --> 00:19:51,887
that's what we moved down to

530
00:19:51,887 --> 00:19:53,111
the next one,

531
00:19:53,112 --> 00:19:54,957
the next largest network which

532
00:19:54,957 --> 00:19:56,258
was 49 hosts.

533
00:19:58,626 --> 00:20:00,151
That requires a subnet mask

534
00:20:00,152 --> 00:20:01,605
of /26.

535
00:20:04,420 --> 00:20:06,541
/26s are always in increments

536
00:20:06,542 --> 00:20:09,996
or multiples of 64.

537
00:20:11,039 --> 00:20:15,816
So we take a 192 plus 64,

538
00:20:15,817 --> 00:20:17,156
and that gives us our next

539
00:20:17,157 --> 00:20:20,105
available network.

540
00:20:20,105 --> 00:20:22,888
I'm sorry, 128 plus 64.

541
00:20:22,889 --> 00:20:24,487
So we take our existing network

542
00:20:24,488 --> 00:20:27,124
of 128, add 64 to it,

543
00:20:27,124 --> 00:20:28,205
and our next available network

544
00:20:28,206 --> 00:20:29,705
was 192.

545
00:20:29,706 --> 00:20:31,395
And so that's where we put on

546
00:20:31,395 --> 00:20:34,654
network number 4.

547
00:20:34,655 --> 00:20:35,721
With 19 hosts,

548
00:20:35,722 --> 00:20:39,619
we use a /27 subnet mask.

549
00:20:39,620 --> 00:20:41,573
/27s are always in increments

550
00:20:41,573 --> 00:20:44,567
of 32.

551
00:20:44,568 --> 00:20:47,563
So we take 192 plus 32.

552
00:20:47,563 --> 00:20:49,506
That gives us 224,

553
00:20:49,506 --> 00:20:50,444
and that gives us our

554
00:20:50,445 --> 00:20:54,157
final network.

555
00:21:01,906 --> 00:21:03,849
And let's do one more of these.

556
00:21:08,624 --> 00:21:09,245
Same thing.

557
00:21:09,246 --> 00:21:10,943
There is your starting network,

558
00:21:10,943 --> 00:21:11,755
there are your

559
00:21:11,756 --> 00:21:12,500
host requirements,

560
00:21:12,501 --> 00:21:13,718
so I'll give you one minute to

561
00:21:13,719 --> 00:21:15,469
work through this question.

562
00:21:24,520 --> 00:21:25,193
All right.

563
00:21:25,194 --> 00:21:26,764
Time's up for this one.

564
00:21:29,241 --> 00:21:30,796
And here are the answers.

565
00:21:30,797 --> 00:21:31,908
So we start with our largest

566
00:21:31,909 --> 00:21:35,003
network here which was the one

567
00:21:35,004 --> 00:21:37,353
requiring 220 hosts.

568
00:21:39,502 --> 00:21:40,814
For that, we actually need a

569
00:21:40,815 --> 00:21:42,907
/24 which supports up to

570
00:21:42,908 --> 00:21:45,104
254 hosts.

571
00:21:47,946 --> 00:21:49,286
So that consumed all of

572
00:21:49,287 --> 00:21:56,621
140.140.0 so 140.140.1 would be

573
00:21:56,622 --> 00:21:59,002
our next network so that's

574
00:21:59,003 --> 00:22:00,160
where we put on network number

575
00:22:00,161 --> 00:22:01,795
3 with 45 hosts.

576
00:22:03,846 --> 00:22:06,282
A /26 is an appropriate subnet

577
00:22:06,283 --> 00:22:09,277
mask to support 45 hosts.

578
00:22:10,861 --> 00:22:11,989
Our networks are always going

579
00:22:11,990 --> 00:22:13,382
to be in multiples or increments

580
00:22:13,383 --> 00:22:16,369
of 64 with a /26.

581
00:22:17,905 --> 00:22:19,126
So we take our existing network

582
00:22:19,127 --> 00:22:21,687
of 1.0, we add 64 to it,

583
00:22:21,688 --> 00:22:25,034
and that gives us 1.64.

584
00:22:25,035 --> 00:22:26,197
That's what we place on our

585
00:22:26,198 --> 00:22:28,030
next smallest network.

586
00:22:30,916 --> 00:22:31,759
Only 6 hosts,

587
00:22:31,760 --> 00:22:33,030
that means we can get away with

588
00:22:33,031 --> 00:22:35,378
a subnet mask of /29.

589
00:22:36,819 --> 00:22:38,766
/29s are always in multiples of

590
00:22:38,767 --> 00:22:45,390
8s, so 64 plus 8 is 72,

591
00:22:45,391 --> 00:22:46,037
which goes on our

592
00:22:46,038 --> 00:22:47,395
final network,

593
00:22:48,600 --> 00:22:50,060
which gives a subnet mask of

594
00:22:50,061 --> 00:22:52,526
/30 based on its

595
00:22:52,527 --> 00:22:53,889
host requirements.

596
00:22:57,187 --> 00:22:59,074
Okay. Let's go back to that one

597
00:22:59,074 --> 00:23:00,851
slide I skipped so that you

598
00:23:00,852 --> 00:23:02,008
understand what that concept is

599
00:23:02,009 --> 00:23:02,665
of classless

600
00:23:02,666 --> 00:23:05,656
interdomain routing.

601
00:23:05,657 --> 00:23:06,696
What's that talking about?

602
00:23:10,018 --> 00:23:12,060
Classless interdomain routing

603
00:23:12,061 --> 00:23:14,070
is just basically a fancy way

604
00:23:14,071 --> 00:23:16,365
of saying that this whole idea

605
00:23:16,366 --> 00:23:18,961
of classes A, B, and C

606
00:23:18,962 --> 00:23:22,001
is now irrelevant.

607
00:23:22,001 --> 00:23:23,094
And what do I mean by that?

608
00:23:23,094 --> 00:23:29,185
Well, if you had TCP/IP

609
00:23:29,186 --> 00:23:31,877
software running on your laptop

610
00:23:31,878 --> 00:23:34,300
or your PC that follow the

611
00:23:34,301 --> 00:23:35,388
classful rules,

612
00:23:37,305 --> 00:23:38,274
it might still let you

613
00:23:38,275 --> 00:23:40,002
do subnetting. Right?

614
00:23:40,003 --> 00:23:41,330
If I start with a

615
00:23:41,331 --> 00:23:43,112
Class B address,

616
00:23:43,113 --> 00:23:45,160
I could put in a subnet mask

617
00:23:45,161 --> 00:23:46,685
that instead of a /16,

618
00:23:46,686 --> 00:23:48,431
which is the classful mask,

619
00:23:49,251 --> 00:23:50,304
it might allow me to put in a

620
00:23:50,305 --> 00:23:52,899
subnet mask of /17, /18,

621
00:23:52,900 --> 00:23:54,469
/30 whatever,

622
00:23:54,470 --> 00:23:55,603
as long as I didn't go less

623
00:23:55,604 --> 00:23:58,468
than /16.

624
00:23:58,468 --> 00:23:59,630
With classless

625
00:23:59,631 --> 00:24:00,263
interdomain routing,

626
00:24:00,264 --> 00:24:01,411
the means the whole idea of

627
00:24:01,412 --> 00:24:04,368
classes is completely gone.

628
00:24:04,369 --> 00:24:05,493
That means you can apply any

629
00:24:05,494 --> 00:24:07,566
subnet mask you want to

630
00:24:07,567 --> 00:24:10,182
any address.

631
00:24:11,493 --> 00:24:14,183
For example,

632
00:24:14,183 --> 00:24:15,498
and you bring up one

633
00:24:15,499 --> 00:24:16,850
right here.

634
00:24:19,909 --> 00:24:21,563
So right now I'm logged in--

635
00:24:21,564 --> 00:24:22,525
this is actually a router.

636
00:24:22,525 --> 00:24:23,451
It's not a host.

637
00:24:25,299 --> 00:24:27,376
But I can go into his interface

638
00:24:29,518 --> 00:24:31,224
and I can say IP address--

639
00:24:33,093 --> 00:24:33,652
let's give him a

640
00:24:33,653 --> 00:24:35,606
Class B address.

641
00:24:35,607 --> 00:24:37,845
Anything that's 128 or greater.

642
00:24:37,846 --> 00:24:38,612
Let's say 130.

643
00:24:41,440 --> 00:24:42,620
Now look at this.

644
00:24:42,621 --> 00:24:44,640
If this was a classful system,

645
00:24:49,505 --> 00:24:51,523
it wouldn't let me do that

646
00:24:51,524 --> 00:24:54,824
because 255.248,

647
00:24:54,824 --> 00:24:55,642
what is that telling

648
00:24:55,643 --> 00:24:56,132
the system?

649
00:24:56,132 --> 00:24:56,567
That's saying,

650
00:24:56,568 --> 00:24:57,975
Okay, of that address I just

651
00:24:57,976 --> 00:25:00,056
gave you,

652
00:25:00,056 --> 00:25:02,702
the first 8 bits - so the

653
00:25:02,703 --> 00:25:04,482
whole number of 130 - is

654
00:25:04,483 --> 00:25:05,864
networking bits,

655
00:25:05,865 --> 00:25:08,083
and of the second byte,

656
00:25:08,084 --> 00:25:10,052
5 of those bits are

657
00:25:10,053 --> 00:25:11,782
networking bits.

658
00:25:11,783 --> 00:25:12,526
So it's basically saying,

659
00:25:12,526 --> 00:25:13,478
Of that number,

660
00:25:13,479 --> 00:25:16,360
13 of the bits,

661
00:25:17,783 --> 00:25:18,516
if I did that right.

662
00:25:18,517 --> 00:25:19,301
8 plus 5.

663
00:25:19,302 --> 00:25:20,977
9, 10, 11, 12, 13. Yeah.

664
00:25:20,978 --> 00:25:23,383
13 bits are networking bits.

665
00:25:23,384 --> 00:25:23,949
If this was a

666
00:25:23,950 --> 00:25:25,075
classful-based system,

667
00:25:25,076 --> 00:25:26,792
it'd say, No, no, no,

668
00:25:26,793 --> 00:25:28,104
you can't do that.

669
00:25:28,105 --> 00:25:30,151
130 falls in the Class B space.

670
00:25:31,236 --> 00:25:32,889
This number,

671
00:25:32,890 --> 00:25:35,341
at least 16 bits have to be

672
00:25:35,342 --> 00:25:36,822
the networking.

673
00:25:36,823 --> 00:25:38,540
You can give me more but at

674
00:25:38,541 --> 00:25:40,370
least 16 of the bits have to be

675
00:25:40,371 --> 00:25:41,325
networking because this is

676
00:25:41,326 --> 00:25:42,613
Class B.

677
00:25:42,614 --> 00:25:45,396
But because this is a classless

678
00:25:45,397 --> 00:25:47,682
interdomain routing device--

679
00:25:49,308 --> 00:25:50,299
I guess it would help if I

680
00:25:50,300 --> 00:25:51,031
actually did it on

681
00:25:51,032 --> 00:25:51,972
an interface.

682
00:25:56,863 --> 00:25:58,335
It takes it.

683
00:25:58,336 --> 00:25:59,896
No problem.

684
00:25:59,897 --> 00:26:00,350
He says, Look,

685
00:26:00,350 --> 00:26:02,109
you can give me any 32-bit

686
00:26:02,110 --> 00:26:02,793
IP address.

687
00:26:02,794 --> 00:26:03,903
I don't care.

688
00:26:03,904 --> 00:26:04,973
As long as you tell me where

689
00:26:04,974 --> 00:26:06,277
the dividing line is between

690
00:26:06,278 --> 00:26:07,547
network and host.

691
00:26:07,548 --> 00:26:08,019
As a matter of fact,

692
00:26:08,020 --> 00:26:08,580
I could do something

693
00:26:08,580 --> 00:26:09,260
real crazy.

694
00:26:09,261 --> 00:26:10,531
Look at this.

695
00:26:10,531 --> 00:26:11,470
IP address.

696
00:26:11,471 --> 00:26:12,093
Let's give him the

697
00:26:12,094 --> 00:26:13,481
same address,

698
00:26:14,942 --> 00:26:16,873
and what if I say--

699
00:26:19,562 --> 00:26:20,126
look at that.

700
00:26:20,127 --> 00:26:21,827
What's that telling him?

701
00:26:21,828 --> 00:26:22,582
That's saying, Oh,

702
00:26:22,583 --> 00:26:24,680
that address that you have,

703
00:26:24,680 --> 00:26:27,003
only the first 2 bits

704
00:26:27,004 --> 00:26:29,671
represent your group.

705
00:26:29,672 --> 00:26:30,945
Everybody in your group has an

706
00:26:30,946 --> 00:26:33,919
address beginning with 10

707
00:26:33,920 --> 00:26:36,236
because if I break 130 down,

708
00:26:36,237 --> 00:26:37,923
it starts with a 120 bit on,

709
00:26:37,924 --> 00:26:39,569
the 64 bit off, 10,

710
00:26:39,570 --> 00:26:40,744
and the subnet mask is saying,

711
00:26:40,745 --> 00:26:42,575
That's all you care about.

712
00:26:42,576 --> 00:26:48,116
Your group is 10 anything.

713
00:26:48,117 --> 00:26:49,317
This is classless

714
00:26:49,318 --> 00:26:50,332
interdomain routing,

715
00:26:50,333 --> 00:26:52,027
where the whole idea of A, B,

716
00:26:52,028 --> 00:26:54,065
or C is completely thrown out

717
00:26:54,066 --> 00:26:55,113
the window.

718
00:26:55,114 --> 00:26:57,871
The subnet mask is in charge of

719
00:26:57,872 --> 00:26:59,314
dictating where the dividing

720
00:26:59,315 --> 00:27:02,066
line is in these bits between

721
00:27:02,067 --> 00:27:04,008
network and host.

722
00:27:04,009 --> 00:27:09,000
[music]