Networking - Subnetting - Discussion

Discussion :: Subnetting - Subnetting (Q.No.4)

4. 

What is the maximum number of IP addresses that can be assigned to hosts on a local subnet that uses the 255.255.255.224 subnet mask?

[A]. 14
[B]. 15
[C]. 16
[D]. 30

Answer: Option D

Explanation:

A /27 (255.255.255.224) is 3 bits on and 5 bits off. This provides 8 subnets, each with 30 hosts. Does it matter if this mask is used with a Class A, B, or C network address? Not at all. The number of host bits would never change.

Nikhil said: (Feb 8, 2014)  
255.255.255.224 i.e Binary of 255 is 11111111 and Binary number of 224 is 11100000.

Here 1 Represent On bit and 0 represents Off bit.
Then how to calculate Host Id.

See Here,

Formula Host Id = 2^off bit - 2 i.e 2^5 = 32 - 2 = 30.
And Net Id is = 2^on bit = 2^3 = 8.
That's why Answer is 30.

Rahul said: (Mar 11, 2015)  
Simple.

= 256-224 = 32.

And here first add is network add and last one is broadcast so we are supposed to subtract these two add from 32. So max host we will connect = 30.

Pratik said: (Jul 8, 2015)  
Thanks @Nikhil but can you explain that binary conversions till 255?

Narsing said: (Jun 16, 2016)  
Decimal to binary conversion:

255.255.255.255

128 64 32 16 8 4 2 1 2 to the power of 7 = 128.

2 to the power of 6 = 64 .

1 1 1 1 1 1 1 1

128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255.

11111111.11111111.11111111.11111111
255 255 255 255

Eg: 192.168.10.10

192-- 128 64 32 16 8 4 2 1
------> 1 1 0 0 0 0 0 0
168 --> 1 0 1 0 1 0 0 0
10 ---> 0 0 0 0 1 0 1 0
10 ---> 0 0 0 0 1 0 1 0

That means where is the 1 is there will add those numbes, so will get decimal number.

Sagar said: (Nov 29, 2016)  
Your answer is easy to understand, Thanks @Nikhil.

Vasundhra said: (Jan 4, 2017)  
Not able to perceive, please explain in an easy way. Is there any formula to calculate subnet mask?

Darth Vader said: (Apr 3, 2017)  
Convert it to binary:

11111111.11111111.11111111.11100000.

You can't use 1s because they are mask bits, you can use 0s as host bits and they can get values between 0 to 31 (total 32 values).

2 of these 32 host values cannot be used as a host IP because one of them belong to broadcast and the other belong to the network address. There is a formula for this host calculation it is 2^(host bits)-2 For this example it is 2^5 - 2 = 32-2 = 30.

Chaithanya said: (Jun 13, 2017)  
24|27 = 3bits on 5 bits off.
2^h-2 h=hostsbits = 5,
32-2 = 30.

Or simple total is 256 so 256-224=30.

Dhananjay said: (Aug 25, 2017)  
Thanks.

Seonti said: (Sep 3, 2017)  
Can anyone explain why it is /27?

Nishu Kumar said: (Sep 24, 2017)  
(1)255.255.255.224
128+64+32=224.

2^3 =8, 2^5-2= 32 -2 =30
(2) formula;

Block size = 256 - Actual subnet mask,
=256 -224,
= 32.

Tara said: (Dec 9, 2017)  
@Seonti.

Here the question is 255.255.255.224.

Convert to binary it is 11111111 11111111 11111111 11100000.
Count nos of 1 ...it is 27.

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