Subnetting Shortcuts

union122

Hi All, What shortcuts do you guys use on a problem like this, instead of working out each mult. choice
answer ! thanks

You are working in a data center environment and are assigned the address range 10.188.31.0/23.
You are asked to develop an IP addressing plan to allow the maximum number of subnets with as many as 30 hosts each.Which IP address range meets these requirements?
A. 10.188.31.0/27
B. 10.188.31.0/26
C. 10.188.31.0/29
D. 10.188.31.0/28
E. 10.188.31.0/25

j-man

It's asking for as many as 30 hosts per so in order to achieve that you will need to use 5 bits which would be 2^5-2 = 30

so the subnet mask would be

11111111.11111111.11111111.11100000 = /27

fadhil

to check ip address with maximum number of host of 30
first remember ipv4 has 32 bits.
when we write /26 or /13 it means from 32 bits there are either 26 bits or 13 bits which are on(ones) while other are of(zeros).
to find number of host of given subnet like(10.188.31.0/27) must consider number of zeros.
that is /27 =11111111.11111111.1111111.11100000
then number of host =2^n -2 where n is number of zeros from subnet mask
to have 30 hosts
2^5 -2= 30 hence only 10.188.31.0/27 satisfy the condition
the answer is A

elderkai

After awhile, you just get used to knowing how many hosts are on each subnet, really.

One shortcut I use if I'm on the last octet, is that I just think of how many users I need then see which binary increment is closest to that. For example:

For 30 hosts, I know that 32 is a binary increment and it's obvious that's the closest. So in my head I'd picture all 8 bits and look at which represents 32(00000000). It's the third bit, so I would just add three to the classful mask /24 to get /27. In reality, this all happens pretty much instantly and I kind of just know what subnets are what, but looking at it like that might help you. ^.^ Hope it was explained clearly enough, though.

union122

thanks all.. i like that elderkai.. quick.

union122

A national retail chain needs to design an IP addressing scheme to support a nationwide network. The company needs a minimum of 300 sub-networks and a maximum of 50 host addresses per subnet. Working with only one Class B address, which of the following subnet masks will support an appropriate addressing scheme? (Choose two)
A – 255.255.255.0
B – 255.255.255.128
C – 255.255.252.0
D – 255.255.255.224
E – 255.255.255.192
F – 255.255.248.0

j-man

How about showing us what you've done to solve this riddle? You have been given all the help you need.

You will never learn how to figure out these types of questions unless you put forth some effort.

So now. Show us your work and then ask questions.

We're here to help you, not to do your work for you.

elderkai

I'm not going to finish it all for you, but I can say that you won't even get 300 subnets period into the fourth octet.

fadhil

union122 wrote: »

A national retail chain needs to design an IP addressing scheme to support a nationwide network. The company needs a minimum of 300 sub-networks and a maximum of 50 host addresses per subnet. Working with only one Class B address, which of the following subnet masks will support an appropriate addressing scheme? (Choose two)
A – 255.255.255.0
B – 255.255.255.128
C – 255.255.252.0
D – 255.255.255.224
E – 255.255.255.192
F – 255.255.248.0

the question is simple since the company needs maximum of 50 hosts and minimum of 300 subnets, first consider on number of host.
we should check on the subnet which will provide 50 hosts i.e 2^n -2 >= 50 where is number of zeros.
hence the subnet that will satisfy the condition is 255.255.255.192 because it has 6 zeros .
so when we find number of hosts 2^6 -2 =62 the maximum number of hosts ( note:if we choose of 5 zeros at it's subnet will get maximum of 30 hosts)
to get number of subnet from class B.
default mask of class B is 255.255.0.0 and the mask is 255.255.255.192, it means that there are 10 bits which are on(ones) that first was off as we on it's default mask
hence subnet will be 2^10 =1024.
hence the answer is E and B since all of them satisfy the requirements of minimum 300 subnet and hosts of 50.

Ltat42a

fadhil wrote: »

hence the answer is E

Be careful here!!! The question stated "Choose two". One of the type of questions that will "getcha" on the exam

union122

Your right j-man
Okay:
giving: 50 hosts 300 networks, class B


50<2^x -2
X=6

so its 32-6= /26

I know how to get the calucations.. but now i would test each choice to see if it fits.. takes too long.
thanks all..

j-man wrote: »

How about showing us what you've done to solve this riddle? You have been given all the help you need.

You will never learn how to figure out these types of questions unless you put forth some effort.

So now. Show us your work and then ask questions.

We're here to help you, not to do your work for you.

poojajoshi

hi i have one subnetting Question which is very confusing pls help soon....!!!!!

Q)you have a /27 subnet mask. which of the following are valid host?(choose three)

1)11.244.18.63
2)90.10.170.93
3)143.187.16.56
4)192.168.15.87
5)200.45.115.159
6)216.66.11.192

Ltat42a

To get your answer, apply the /27 to each of the addresses. Is the address a network address or broadcast address? If not, the address would be valid.

hth

oli356

Find out what the network increment is for /27 (is the network going up by 8 e.g. 192.168.0.0, 192.168.8.0, 192.168.16.0... etc
And what octet we're subnetting in.

I just did this in my head by going "Well that one is a broadcast address so it can't be that one, that is a network address so no.... " Just counted up the network increment.
Good luck