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

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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.

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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?

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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

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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,

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Imran.
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1. C
2. A

MS is right about #2 and explained it beautifully.

JBGood

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1.c
2.a

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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

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quote:

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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

---

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Last edited by ms on 12-28-01 at 08:23 AM

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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 :

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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.

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