CCNA - Subnetting

Author

Subnetting

bfattima

2001-12-27, 11:16 am

1- A router received a packet with a destination address of 172.16.14.228 assuming a subnet mask of 255.255.248.0 what is the subnet network address for the destination host?
A- 172.16.1.0
B- 172.16.4.0
C- 172.16.8.0
D- 172.16.12.0

2- If a router has a serial interface S0, with IP address 107.85.20.2, using a subnet mask of 255.255.255.240, what is the broadcast address used for hosts on this subnet?
A- 107.85.20.15
B- 107.85.20.255
C- 107.85.255.255
D- 107.255.255.255
ast ime ai posted but i missed the answers
Thank you

Dude

2001-12-27, 12:50 pm

My guess is as follows:

1. My guess is C. 256-248=8, which is your first subnet. It goes 8, then 16. Boom, your number is 14, so since it is below 16, it must be on subnet 172.16.8.0

2. Not real sure about 2. My guess would be D, but again, I cant explain why.

Dude

2001-12-27, 1:31 pm

Just thinking about your #2 question, can that be a legal address? Im thinking that the first subnet should start with 16, being 107.85.20.16. Guys, am I right in thinking that? or is that wrong?

2001-12-27, 3:48 pm

If a router has a serial interface S0, with IP address 107.85.20.2, using a subnet mask of 255.255.255.240, what is the broadcast address used for hosts on this subnet?
A- 107.85.20.15
B- 107.85.20.255
C- 107.85.255.255
D- 107.255.255.255

The answer is (A) 107.85.20.15 the subnet is 107.85.20.0
remember this is a class A address there are 20 bits borrowed so has long the subnet address is not 107.255.255.0 the last octet can be 0.There are 4 bits for the host so that makes 14 hosts 0 (it is the subnet)cannot be used and 15 is the brodcast address and the host range will be 107.85.20.1 - 107.85.20.14

Remember this is diffrent if it had been a class c address.

by the way they don't make it this tough in the exam

Imran4sin

2001-12-27, 5:54 pm

Sup,
For #1) I will go for C; here is how;
Using Anding we get;
if its
1
1, it becomes=>1,
if its
1
0, it becomes=>0.
Hence using anding on all the numbers we get=>
172.16.14.228 --1
255.255.248.0 --2

10101100.00010000.00001110.11100100--1
11111111.11111111.11111000.00000000--2 anding 1 and 2 gives
10101100.00010000.00001000.00000000>binary
172 .16 .8 .0 >decimal
Hence we get
172.16.8.0--C

For #2) I will go for D;
But im not sure about this oone though--i might agree with ms..havent given it much thought .

Hope this helps,
l8r,

JohnnyBeGood

2001-12-27, 7:03 pm

1. C
2. A

MS is right about #2 and explained it beautifully.

JBGood

VaughanW

2001-12-27, 7:09 pm

1.c
2.a

Hippo

2001-12-28, 1:01 am

What has been omitted here is the explanation that although the address 107.85.20.2 belongs in subnet 107.85.20.0, subnet zero addresses can only be used if the router command 'ip subnet-zero' has been issued.

hippo

2001-12-28, 3:20 am

quote:
What has been omitted here is the explanation that although the address 107.85.20.2 belongs in subnet 107.85.20.0, subnet zero addresses can only be used if the router command 'ip subnet-zero' has been issued

The ip subnet-zero command is used when you borrow one bit(because 2^1 -2=0)so for example if I had a class c address :

200.16.10.0
255.255.255.128

since only one bit is borrowed which means i have 0 subnets this would be illegal. the way to get round this is by using the
ip subnet-zero command and this allows you to create 2 subnets out off 1 bit which you borrowed.

so when you see a subnet mask like the above example then they will be two networks
with 126 hosts in each subnet(2^7-2=126)

subnet: 200.16.10.0
host range: 200.16.10.1-200.16.10.126
broadcast address: 200.16.10.127

subnet: 200.16.10.128
host range: 200.16.10.129-200.16.10.254
broadcast address: 200.16.10.255

you have to know this for the exam

dmaftei

2001-12-28, 8:18 am

quote:
Originally posted by ms
The ip subnet-zero command is used when you borrow one bit(because 2^1 -2=0)so for example if I had a class c address :

Not so. ip subnet-zero is used when one intends to use the first subnet, no matter how many bits one borrows. If one subnets 192.168.1.0 with mask 255.255.255.224, one gets the following subnets:

192.168.1.0 <-- first subnet, one needs to use ip subnet-zero
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

And, btw, in the latest versions of IOS ip subnet-zero in on by default.

Hippo, old buddy, the first subnet (subnet-zero) for 107.85.20.2 mask 255.255.255.240 is 107.0.0.0. 107.85.20.0/28 is in the neighborhood of the 1700th subnet.

Hippo

2001-12-28, 8:30 am

Thanks Dan

Looks like I got bit rusty with subnetting.

Cheers
Hippo

2001-12-28, 9:11 am

[QUOTE]Originally posted by dmaftei

Not so. [b]ip subnet-zero is used when one intends to use the first subnet, no matter how many bits one borrows. If one subnets 192.168.1.0 with mask 255.255.255.224, one gets the following subnets:

192.168.1.0 <-- first subnet, one needs to use ip subnet-zero
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

And, btw, in the latest versions of IOS ip subnet-zero in on by default.

ok if you use the above example I have borrowed 3 bits so that gives me 6 subnets(2^3-2) but what you have shown there will be 8 subnets
192.168.1.0
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224 :- and in this one the host range will be 192.168.1.225 - 192.168.1.254 and when I do an and it works out that the subnetmask should be 255.255.255.240

my calculations lead me to the following 6 subnets :

192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192

dmaftei

2001-12-28, 11:09 am

quote:
Originally posted by ms
ok if you use the above example I have borrowed 3 bits so that gives me 6 subnets(2^3-2) but what you have shown there will be 8 subnets

You just made the point I've always maintained, that the 2^n-2 rule is BAD!! Suppose you subnet a class C net, say 192.168.1.0 -- that's 8 bits in the host portion of the address. You borrow 3 bits for subnets, and you're left with 5 bits in the host portion. That gives you 8 subnets (2^3 = 8), and 32 addresses per subnet (2^5 = 32).

Now, why the first and last subnets are not used is a long story that goes to the early ages of the Internet. I am aware that for CCNA purposes the first and last subnets are not to be used. However, this does not mean that the first and last subnets do not exist!! They do exist (and in real life they can be and are used).

As for the "addresses per subnet" part, the 2^n-2 rule has some merit, in that it states "there are 2^n-2 host addresses per subnet". That's true, since of the 2^n addresses the first one is used to denote the subnet itself (the wire), while the last one is the broadcast for that subnet. That leaves you with 2^n-2 (30 in our example) addresses that can be assigned to hosts.

Hope this clears some confusion.

2001-12-28, 12:01 pm

quote:
You just made the point I've always maintained, that the 2^n-2 rule is BAD!! Suppose you subnet a class C net, say 192.168.1.0 -- that's 8 bits in the host portion of the address. You borrow 3 bits for subnets, and you're left with 5 bits in the host portion. That gives you 8 subnets (2^3 = 8), and 32 addresses per subnet (2^5 = 32).

I don't want to mess up the ccna students so it might be better for them not to get so deep into this discussion.

I am inserting the text from cisco press ccnp routing page 104-105:-

Consideration must be given to the subnetting rules (RFC 950, “Internet Standard Subnetting
Procedure,” and rfc 1878, “Variable-Length Subnet Table For IPv4”) that state that there must not be all zeros or all ones in the following:
• The NIC portion of the address
• The subnet portion of the address
• The host portion of the address
The algorithm for calculating the number of networks or hosts available is 2n – 2 (where n is the number of bits by which the subnet mask was extended).

This rule has become complicated recently regarding the subnet portion of the address. The number of subnets is still calculated by the 2n formula, where n is the number of bits by which the subnet mask was extended. However, it is possible to use the all-zero address for the subnet.
This makes the formula 2n – 1.

WARNING: Although Cisco provides the utility of subnet zero, this command should be used only with full understanding of the network devices and the knowledge that there is no device that uses the zero broadcast. Even today, some systems, such as Sun Solaris 4.x, have problems using subnet zero even with OSPF.

To keep it simple for ccna exam takers the formula is still 2^n-2

I hope I havent confused any of the ccna students further.

ROUTERRIP

2001-12-31, 11:36 am

The fact of the matter is that the Cisco Press CCNA guide (W. Odom) preaches 2n for number of subnets and 2n-2 for number of hosts.

Go figure. The "CCNA for dummies" book uses 2n-2 for both.

I trust the Cisco Press book, and that's what I'm doing for the test.

255.255.240.0 for a class B address = 16 subnets!!!

255.255.0.0
255.255.16.0
255.255.32.0
255.255.48.0
255.255.64.0
.
.
.
255.255.240.0

dmaftei

2002-01-05, 11:06 pm

quote:
Originally posted by ms
... Consideration must be given to the subnetting rules (RFC 950, “Internet Standard Subnetting
Procedure,” and rfc 1878, “Variable-Length Subnet Table For IPv4”)...

Did you actually read these RFCs, or you're just qouting from the book?!

quote:
I hope I havent confused any of the ccna students further.

What confuses ccna students -- and any student, for that matter -- is not understanding what they're studying. Any effort to clarify things should be welcome...

2002-01-06, 4:31 am

quote:
Originally posted by dmaftei

Did you actually reaf these RFCs, or you're just qouting from the book?!

What confuses ccna students -- and any student, for that matter -- is not understanding what they're studying. Any effort to clarify things should be welcome...

I an actually quoting from the ccnp cisco press book.

I was speaking to some mcse students and they also have the same problem that is which rule to follow.

The only thing I can say is after you see the ccna exam questions the answers seem to indicate that they use the 2^n-2 rule.

dmaftei

2002-01-06, 10:59 am

quote:
Originally posted by ms
I an actually quoting from the ccnp cisco press book.

Then give the RFCs a try, they're instuctive: http://www.rfc-editor.org/rfcsearch.html

quote:
The only thing I can say is after you see the ccna exam questions the answers seem to indicate that they use the 2^n-2 rule.

Yeah, I know CCNA ignores the first and last subnets. But the total number of subnets is still 2^n.

- for hosts you get 2^n-2 host addresses, plus 1 (one) subnet address, plus 1 (one) broadcast address, for a total of 2^n - 2 + 1 + 1 = 2^n addresses per subnet.

- for subnets you get 2^n-2 always-usable-without-any-problem subnets, plus 1 (one) subnet-zero subnet, plus 1 (one) all-ones subnet, for a total of 2^n - 2 + 1 + 1 = 2^n subnets.

2002-01-06, 11:14 am

quote:
for subnets you get 2^n-2 always-usable-without-any-problem subnets, plus 1 (one) subnet-zero subnet, plus 1 (one) all-ones subnet, for a total of 2^n - 2 + 1 + 1 = 2^n subnets.

Thanks I got what you are trying to say but in which cisco exam would i be using the 2^n
formula

dmaftei

2002-01-06, 12:24 pm

quote:
Originally posted by ms
... but in which cisco exam would i be using the 2^n formula

I don't know. I do know that for CCNA the first and last subnets are to be ignored. For other exams, as well as for real life, I would guess one needs more info, like "are all the hosts in the domain one is subnetting able to use all subnets?" and such...

catfisch

2002-01-06, 2:32 pm

Dylon

2002-01-10, 2:35 pm

I would say for No.1, C
and for No.2, A. It's a class A address.

freak

2002-01-11, 10:03 am

for help with subnetting, visit www.mcsefreak.com/subnetting.htm
I hope this can help!

Boredsilly

2002-01-12, 12:26 am

1. A subnet mask of 255.255.248.0 would mean you have networks that are (256-248=8) 8 class C's in size. The first and second subnet ranges would therefore be:

172.16.0.0 to 172.16.7.255
172.16.8.0 to 172.16.15.255

An IP address of 172.16.14.228 is found in the second subnet, which begins with network address of 172.16.8.0. I believe the answer is C.

2. Using the same sort of logic, we see that a subnet mask of 255.255.255.240 would give us networks that are (256-240=16) 16 IP addresses in size (we are dealing in IP addresses rather than class C's now because the 255 value fills 3 octets in this case). The IP address in question belong to the first subnet:

107.85.20.0 to 107.85.20.15

The broadcast address of this subnet is 107.85.20.15. I believe the answer is A.

six76

2002-01-12, 9:29 am

Agree with ms. 1c, 2a.

I think that dmaftei is also right.

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