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2009-06-16T10:40:00.000-07:00

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Subnetting in Your Head: Class B Addresses

2009-02-26T23:25:00.001-08:00

Are you nuts? Subnet Class B addresses in our heads? It’s absolutely easier than autograph it out—

I’m not kidding! Let me appearance you how:

Question: What subnet and advertisement abode is the IP abode 172.16.10.33

255.255.255.224 (/27) a affiliate of?

Answer: The absorbing octet is the fourth octet. 256 – 224 = 32. 32 + 32 = 64. Bingo: 33

is amid 32 and 64. However, bethink that the third octet is advised allotment of the

subnet, so the acknowledgment would be the 10.32 subnet. The advertisement is 10.63, back 10.64 is

the abutting subnet. That was a appealing accessible one.

Question: What subnet and advertisement abode is the IP abode 172.16.66.10

255.255.192.0 (/18) a affiliate of?

Answer: The absorbing octet is the third octet instead of the fourth octet. 256 – 192 = 64.

0, 64, 128. The subnet is 172.16.64.0. The advertisement charge be 172.16.127.255 since

128.0 is the abutting subnet.

Question: What subnet and advertisement abode is the IP abode 172.16.50.10

255.255.224.0 (/19) a affiliate of?

Answer: 256 – 224 = 0, 32, 64 (remember, we consistently alpha counting at aught (0)). The subnet

is 172.16.32.0, and the advertisement charge be 172.16.63.25 back 64.0 is the abutting subnet.

Question: What subnet and advertisement abode is the IP abode 172.16.46.255

255.255.240.0 (/20) a affiliate of?

Answer: 256 – 240 = 16. The third octet is absorbing to us. 0, 16, 32, 48. This subnet

address charge be in the 172.16.32.0 subnet, and the advertisement charge be 172.16.47.255

since 48.0 is the abutting subnet. So, yes, 172.16.46.255 is a accurate host.

Question: What subnet and advertisement abode is the IP abode 172.16.45.14

255.255.255.252 (/30) a affiliate of?

Answer: Where is the absorbing octet? 256 – 252 = 0, 4, 8, 12, 16 (in the fourth octet).

The subnet is 172.16.45.12, with a advertisement of 172.16.45.15 because the abutting subnet is

172.16.45.16.

Question: What is the subnet and advertisement abode of the host 172.16.88.255/20?

Answer: What is a /20? If you can’t acknowledgment this, you can’t acknowledgment this question, can you?

A /20 is 255.255.240.0, which gives us a block admeasurement of 16 in the third octet, and back no

subnet $.25 are on in the fourth octet, the acknowledgment is consistently 0 and 255 in the fourth octet.

0, 16, 32, 48, 64, 80, 96…bingo. 88 is amid 80 and 96, so the subnet is 80.0 and the

broadcast abode is 95.255.

Question: A router receives a packet on an interface with a destination abode of

172.16.46.191/26. What will the router do with this packet?

Answer: Abandon it. Do you apperceive why? 172.16.46.191/26 is a 255.255.255.192 mask,

which gives us a block admeasurement of 64. Our subnets are again 0, 64, 128, 192. 191 is the broadcast

address of the 128 subnet, so a router, by default, will abandon any advertisement packets.

Practice Example #8B: 255.255.255.224 (/27)

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This is done the aforementioned way as the above-mentioned subnet mask, except that we aloof accept added subnets

and beneath hosts per subnet available.

172.16.0.0 = Network address

255.255.255.224 = Subnet mask

Subnets? 211 = 2048.

Hosts? 25 – 2 = 30.

Valid subnets? 256 – 224 = 32. 0, 32, 64, 96, 128, 160, 192, 224.

Broadcast abode for anniversary subnet?

Valid hosts?

The afterward table shows the aboriginal eight subnets:

Subnet 0.0 0.32 0.64 0.96 0.128 0.160 0.192 0.224

First host 0.1 0.33 0.65 0.97 0.129 0.161 0.193 0.225

Last host 0.30 0.62 0.94 0.126 0.158 0.190 0.222 0.254

Broadcast 0.31 0.63 0.95 0.127 0.159 0.191 0.223 0.255

This abutting table shows the aftermost eight subnets:

Subnet 255.0 255.32 255.64 255.96 255.128 255.160 255.192 255.224

First host 255.1 255.33 255.65 255.97 255.129 255.161 255.193 255.225

Last host 255.30 255.62 255.94 255.126 255.158 255.190 255.222 255.254

Broadcast 255.31 255.63 255.95 255.127 255.159 255.191 255.223 255.255

Practice Example #7B: 255.255.255.192 (/26)

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Now, this is area Class B subnetting gets easy. Since the third octet has a 255 in the mask

section, whatever cardinal is listed in the third octet is a subnet number. However, now that

we accept a subnet cardinal in the fourth octet, we can subnet this octet aloof as we did with

Class C subnetting. Let’s try it out:

172.16.0.0 = Network address

255.255.255.192 = Subnet mask

Subnets? 210 = 1024.

Hosts? 26 – 2 = 62.

Accurate subnets? 256 – 192 = 64. The subnets are apparent in the afterward table. Do these

numbers attending familiar?

Broadcast abode for anniversary subnet?

Accurate hosts?

The afterward table shows the aboriginal eight subnet ranges, accurate hosts, and broadcast

addresses:

Subnet 0.0 0.64 0.128 0.192 1.0 1.64 1.128 1.192

First host 0.1 0.65 0.129 0.193 1.1 1.65 1.129 1.193

Last host 0.62 0.126 0.190 0.254 1.62 1.126 1.190 1.254

Broadcast 0.63 0.127 0.191 0.255 1.63 1.127 1.191 1.255

Notice that for anniversary subnet amount in the third octet, you get subnets 0, 64, 128, and 192 in

the fourth octet.

Practice Archetype #6B: 255.255.255.128 (/25)

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This is one of the hardest subnet masks you can comedy with. And worse, it absolutely is a really

good subnet to use in assembly because it creates over 500 subnets with 126 hosts for each

subnet—a nice mixture. So, don’t skip over it!

172.16.0.0 = Network address

255.255.255.128 = Subnet mask

Subnet 0.0 2.0 4.0 6.0 8.0

First host 0.1 2.1 4.1 6.1 8.1

Last host 1.254 3.254 5.254 7.254 9.254

Broadcast 1.255 3.255 5.255 7.255 9.255

Subnet 0.0 1.0 2.0 3.0 ... 254.0 255.0

First host 0.1 1.1 2.1 3.1 ... 254.1 255.1

Last host 0.254 1.254 2.254 3.254 ... 254.254 255.254

Broadcast 0.255 1.255 2.255 3.255 ... 254.255 255.255

Subnetting Basics 131

Subnets? 29 = 512.

Hosts? 27 – 2 = 126.

Accurate subnets? Okay, now for the catchy part. 256 – 255 = 1. 0, 1, 2, 3, etc. for the third

octet. But you can’t balloon the one subnet bit acclimated in the fourth octet. Remember back I

showed you how to amount one subnet bit with a Class C mask? You amount this the same

way. (Now you apperceive why I showed you the 1-bit subnet affectation in the Class C section—

to accomplish this allotment easier.) You absolutely get two subnets for anniversary third octet value, hence

the 512 subnets. For example, if the third octet is assuming subnet 3, the two subnets

would absolutely be 3.0 and 3.128.

Broadcast abode for anniversary subnet?

Accurate hosts?

The afterward table shows how you can actualize subnets, accurate hosts, and broadcast

addresses application the Class B 255.255.255.128 subnet affectation (the aboriginal eight subnets are shown,

and again the aftermost two subnets):

Subnet 0.0 0.128 1.0 1.128 2.0 2.128 3.0 3.128 ... 255.0 255.128

First

host

0.1 0.129 1.1 1.129 2.1 2.129 3.1 3.129 ... 255.1 255.129

Last

host

0.126 0.254 1.126 1.254 2.126 2.254 3.126 3.254 ... 255.126 255.254

Broadcast

0.127 0.255 1.127 1.255 2.127 2.255 3.127 3.255 ... 255.127 255.255

Practice Example #5B: 255.255.255.0 (/24)

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Contrary to accustomed belief, 255.255.255.0 acclimated with a Class B adjustment address is not declared a

Class B adjustment with a Class C subnet mask. It’s amazing how abounding bodies see this affectation used

in a Class B adjustment and ahead it’s a Class C subnet mask. This is a Class B subnet affectation with

8 $.25 of subnetting—it’s appreciably adapted from a Class C mask. Subnetting this address is

fairly simple:

172.16.0.0 = Adjustment address

255.255.255.0 = Subnet mask

Subnets? 28 = 256.

Hosts? 28 – 2 = 254.

Authentic subnets? 256 – 255 = 1. 0, 1, 2, 3, etc., all the way to 255.

Broadcast address for ceremony subnet?

Authentic hosts?

The after table shows the ancient four and aftermost two subnets, the authentic hosts, and the

broadcast addresses in a Class B 255.255.255.0 mask:

Subnet 0.0 1.0 2.0 3.0 ... 254.0 255.0

First host 0.1 1.1 2.1 3.1 ... 254.1 255.1

Last host 0.254 1.254 2.254 3.254 ... 254.254 255.254

Broadcast 0.255 1.255 2.255 3.255 ... 254.255 255.255

Practice Example #4B: 255.255.254.0 (/23)

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Subnets? 27 = 128.
Hosts? 29 – 2 = 510.
Accurate subnets? 256 – 254 = 0, 2, 4, 6, 8, etc., up to 254.
Advertisement abode for anniversary subnet?
Accurate hosts?

172.16.0.0 = Network address

255.255.254.0 = Subnet mask

Subnet 0.0 64.0 128.0 192.0

First host 0.1 64.1 128.1 192.1

Last host 63.254 127.254 191.254 255.254

Broadcast 63.255 127.255 191.255 255.255

Subnet 0.0 16.0 32.0 48.0

First host 0.1 16.1 32.1 48.1

Last host 15.254 31.254 47.254 63.254

Broadcast 15.255 31.255 47.255 63.255

130 Chapter 3 Subnetting, VLSMs, and Troubleshooting TCP/IP

The afterward table shows the aboriginal bristles subnets, accurate hosts, and advertisement addresses in a

Class B 255.255.254.0 mask:

Subnet 0.0 2.0 4.0 6.0 8.0

First host 0.1 2.1 4.1 6.1 8.1

Last host 1.254 3.254 5.254 7.254 9.254

Broadcast 1.255 3.255 5.255 7.255 9.255

Practice Example #3B: 255.255.240.0 (/20)

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Subnets? 24 = 16.
Hosts? 212 – 2 = 4094.
Accurate subnets? 256 – 240 = 0, 16, 32, 48, etc., up to 240. Notice that these are the same
Advertisement abode for anniversary subnet?
Accurate hosts?

172.16.0.0 = Network address

255.255.240.0 = Subnet mask

numbers as a Class C 240 affectation – we aloof put them in the third octet and add a 0 and 255

in the fourth octet.

The afterward table shows the aboriginal four subnets, accurate hosts, and advertisement addresses in a

Class B 255.255.240.0 mask:

Subnet 0.0 16.0 32.0 48.0

First host 0.1 16.1 32.1 48.1

Last host 15.254 31.254 47.254 63.254

Broadcast 15.255 31.255 47.255 63.255

Practice Example #2B: 255.255.192.0 (/18)

2009-02-14T11:35:00.001-08:00

Subnets? 22 = 4.
Hosts? 214 – 2 = 16,382 (6 $.25 in the third octet, and 8 in the fourth).
Accurate subnets? 256 – 192 = 64. 0, 64, 128, 192. Remember that the subnetting is performed
Broadcast abode for anniversary subnet?
Accurate hosts?

172.16.0.0 = Network address

255.255.192.0 = Subnet mask

in the third octet, so the subnet numbers are absolutely 0.0, 64.0, 128.0, and 192.0,

as apparent in the abutting table.

Subnet 0.0 128.0

First host 0.1 128.1

Last host 127.254 255.254

Broadcast 127.255 255.255

Subnetting Basics 129

The afterward table shows the four subnets available, the accurate host range, and the broadcast

address of each:

Again, it’s appealing abundant the aforementioned as it is for a Class C subnet—we aloof added 0 and 255 in

the fourth octet for anniversary subnet in the third octet.

Subnet 0.0 64.0 128.0 192.0
First host 0.1 64.1 128.1 192.1
Last host 63.254 127.254 191.254 255.254
Broadcast 63.255 127.255 191.255 255.255

Practice Example #1B: 255.255.128.0 (/17)

2009-02-14T11:34:00.002-08:00

Subnets? 21 = 2 (same as Class C).
Hosts? 215 – 2 = 32,766 (7 $.25 in the third octet, and 8 in the fourth).
Accurate subnets? 256 – 128 = 128. 0, 128. Remember that subnetting is performed in the

172.16.0.0 = Arrangement address

255.255.128.0 = Subnet mask

third octet, so the subnet numbers are absolutely 0.0 and 128.0, as apparent in the abutting table.

These are the exact numbers we acclimated with Class C; we use them in the third octet and add

a 0 in the fourth octet for the arrangement address.

Broadcst abode for anniversary subnet?
Accurate hosts?

The afterward table shows the two subnets available, the accurate host range, and the broadcast

address of each:

Okay, apprehension that we aloof added the fourth octet’s everyman and accomplished ethics and came up

with the answers. And again, it’s done absolutely the aforementioned way as for a Class C subnet. We just

use the aforementioned numbers in the third octet and added 0 and 255 in the fourth octet—pretty simple

huh! I absolutely can’t say this enough: It’s aloof not hard; the numbers never change; we aloof use

them in altered octets!

Subnetting Convenance Examples: Class B Addresses

2009-02-14T11:34:00.001-08:00

This area will accord you an befalling to convenance subnetting Class B addresses. Again, I

have to acknowledgment that this is the aforementioned as subnetting with Class C, except we alpha in the third

octet—with the exact aforementioned numbers!

Subnetting Chic B Addresses

2009-02-14T11:33:00.000-08:00

Before we dive into this, let’s attending at all the accessible Chic B subnet masks first. Apprehension that

we accept a lot added accessible subnet masks than we do with a Chic C arrangement address:

255.255.0.0 (/16)

255.255.128.0 (/17) 255.255.255.0 (/24)

255.255.192.0 (/18) 255.255.255.128 (/25)

255.255.224.0 (/19) 255.255.255.192 (/26)

255.255.240.0 (/20) 255.255.255.224 (/27)

255.255.248.0 (/21) 255.255.255.240 (/28)

255.255.252.0 (/22) 255.255.255.248 (/29)

255.255.254.0 (/23) 255.255.255.252 (/30)

We apperceive the Chic B arrangement abode has 16 $.25 accessible for host addressing. This agency we

can use up to 14 $.25 for subnetting (because we accept to leave at atomic 2 $.25 for host addressing).

Using a /16 agency you are not subnetting with chic B, but it is a affectation you can use.

By the way, do you apprehension annihilation absorbing about that account of subnet values—

a pattern, maybe? Ah ha! That’s absolutely why I had you acquire the

binary-to-decimal numbers at the alpha of this section. Since subnet

mask $.25 alpha on the larboard and move to the appropriate and $.25 can’t be skipped, the

numbers are consistently the aforementioned behindhand of the chic of address. Memorize

this pattern.

The action of subnetting a Chic B arrangement is appealing abundant the aforementioned as it is for a Chic C,

except that you aloof accept added host $.25 and you alpha in the third octet.

Use the aforementioned subnet numbers for the third octet with Chic B that you acclimated for the fourth

octet with Chic C, but add a aught to the arrangement allocation and a 255 to the advertisement section

in the fourth octet. The afterward table shows you an archetype host ambit of two subnets used

in a Chic B 240 (/20) subnet mask:

Just add the accurate hosts amid the numbers, and you’re set!

The above-mentioned archetype is accurate alone until you get up to /24. After that, it’s

numerically absolutely like Chic C.

What Do We Know?

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Okay—here’s area you can absolutely administer what you’ve abstruse so far, and activate committing

it all to memory. This is a actual air-conditioned area that I’ve been application in my classes for years. It will

really advice you attach bottomward subnetting!

When you see a subnet affectation or carve characters (CIDR), you should apperceive the following:

/25 What do we apperceive about a /25?

128 mask
1 $.25 on and 7 $.25 off (10000000)
Block admeasurement of 128
2 subnets, anniversary with 126 hosts

/26 What do we apperceive about a /26

192 mask
2 $.25 on and 6 $.25 off (11000000)
Block admeasurement of 64
4 subnets, anniversary with 62 hosts
224 mask
3 $.25 on and 5 $.25 off (11100000)
Block admeasurement of 32
8 subnets, anniversary with 30 hosts

/27 What do we apperceive about a /27?

/28 What do we apperceive about a /28

240 mask
4 $.25 on and 4 $.25 off
Block admeasurement of 16
16 subnets, anniversary with 14 hosts
248 mask
5 $.25 on and 3 $.25 off
Block admeasurement of 8
32 subnets, anniversary with 6 hosts

/29 What do we apperceive about a /29?

/30 What do we apperceive about a /30?

252 mask
6 $.25 on and 2 $.25 off
Block admeasurement of 4
64 subnets, anniversary with 2 hosts

Regardless of whether you accept a Class A, Class B, or Class C address, the /30 affectation will

provide you with alone two hosts, ever. This affectation is ill-fitted about exclusively—as able-bodied as suggested

by Cisco—for use on point-to-point links.

If you can acquire this “What Do We Know?” section, you’ll be abundant bigger off in your

day-to-day job and in your studies. Try adage it out loud, which helps you acquire things—

yes, your cogent added and/or coworkers will anticipate you’ve absent it, but they probably

already do if you are in the networking field. And if you’re not yet in the networking acreage but

are belief all this to breach into it, you ability as able-bodied accept bodies alpha cerebration you’re an odd

bird now back they will eventually anyway.

Subnetting Basics 127

It’s additionally accessible to address these on some blazon of flashcards and accept bodies analysis your skill.

You’d be afraid at how fast you can get subnetting bottomward if you acquire block sizes as well

as this “What Do We Know?” section.

Subnetting in Your Head: Class C Addresses

2009-02-08T15:46:00.001-08:00

It absolutely is accessible to subnet in your head. Even if you don’t accept me, I’ll appearance you how.

And it’s not all that adamantine either—take the afterward example:

192.168.10.33 = Bulge address

255.255.255.224 = Subnet mask

Subnet 0 4 8 12 … 240 244 248 252

First host 1 5 9 13 … 241 245 249 253

Last host 2 6 10 14 … 242 246 250 254

Broadcast 3 7 11 15 … 243 247 251 255

Should We Absolutely Use This Affectation That Provides Alone Two Hosts?

You are the arrangement ambassador for Acme Corporation in San Francisco, with dozens of

WAN links abutting to your accumulated office. Appropriate now your arrangement is a classful network,

which agency that the aforementioned subnet affectation is on anniversary host and router interface. You’ve read

about classless acquisition breadth you can accept altered admeasurement masks but don’t apperceive what to use

on your point-to-point WAN links. Is the 255.255.255.252 (/30) a accessible affectation in this situation?

Yes, this is a actual accessible affectation in advanced breadth networks.

If you use the 255.255.255.0 mask, again anniversary arrangement would accept 254 hosts, but you alone use 2

addresses with a WAN link! That is a decay of 252 hosts per subnet. If you use the 255.255.255.252

mask, again anniversary subnet has alone 2 hosts and you don’t decay adored addresses. This is a really

important subject, one that we’ll abode in a lot added detail in the area on VLSM network

design after in this chapter.

Subnetting Basics 125

First, actuate the subnet and advertisement abode of the aloft IP address. You can do this

by answering catechism 3 of the big bristles questions: 256 – 224 = 32. 0, 32, 64. The abode of

33 avalanche amid the two subnets of 32 and 64 and charge be allotment of the 192.168.10.32 subnet.

The abutting subnet is 64, so the advertisement abode of the 32 subnet is 63. (Remember that the

broadcast abode of a subnet is consistently the cardinal appropriate afore the abutting subnet.) The valid

host ambit is 33–62 (the numbers amid the subnet and advertisement address). This is too easy!

Okay, let’s try addition one. We’ll subnet addition Class C address:

192.168.10.33 = Bulge address

255.255.255.240 = Subnet mask

What subnet and advertisement abode is the aloft IP abode a affiliate of? 256 – 240 = 16.

0, 16, 32, 48. Bingo—the host abode is amid the 32 and 48 subnets. The subnet is

192.168.10.32, and the advertisement abode is 47 (the abutting subnet is 48). The accurate host range

is 33–46 (the numbers amid the subnet cardinal and the advertisement address).

Okay, we charge to do more, aloof to accomplish abiding you accept this down.

You accept a bulge abode of 192.168.10.174 with a affectation of 255.255.255.240. What is the

valid host range?

The affectation is 240, so we’d do a 256 – 240 = 16. This is our block size. Aloof accumulate abacus 16

until we canyon the host abode of 174, starting at zero, of course: 0, 16, 32, 48, 64, 80, 96, 112,

128, 144, 160, 176. The host abode of 174 is amid 160 and 176, so the subnet is 160. The

broadcast abode is 175; the accurate host ambit is 161–174. That was a boxy one.

One more—just for fun. This is the easiest one of all Class C subnetting:

192.168.10.17 = Bulge address

255.255.255.252 = Subnet mask

What subnet and advertisement abode is the aloft IP abode a affiliate of? 256 – 252 = 0

(always alpha at aught unless told otherwise), 4, 8, 12, 16, 20, etc. You’ve got it! The host address

is amid the 16 and 20 subnets. The subnet is 192.168.10.16, and the advertisement abode is 19.

The accurate host ambit is 17–18.

Now that you’re all over Class C subnetting, let’s move on to Class B subnetting. But before

we do, let’s accept a quick review.

Should We Really Use This Affectation That Provides Alone Two Hosts?

2009-02-08T15:45:00.000-08:00

You are the arrangement ambassador for Acme Corporation in San Francisco, with dozens of

WAN links abutting to your accumulated office. Right now your arrangement is a classful network,