geeksforgeeks Computer NEtwork IP adressing

Question

My answer is 255.255.255.224 but answer given there was A

Answer 1

Using Subnet Zero and the All-Ones Subnet

It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet have always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS Software Release 12.0. Even prior to Cisco IOS Software Release 12.0, subnet zero could be used by entering the ip subnet-zero global configuration command.

On the issue of using subnet zero and the all-ones subnet, RFC 1878  states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.

http://www.ietf.org/rfc/rfc1878.txt

http://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/13711-40.html

so both 255.255.255.224 and 255.255.255.240 are valid ans and they will not give both together

Comments

is this reason be right?

since, address starts with 201 then it is class C address,
In class C NID=24 bits thats why subnet mask starts with 255.255.255
Now as we know direct broadcast address is last address of subnet, here it is 201.15.16.31 that means each subnet contains 2^5=32 host.
that conclude HID=5 bits. so Subnet mask contains 27 1's and 5 0's so
ans is 255.255.255.224. hence D is ans

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yes 1st octet 201 implies class C, so to create subnet we need to borrow some host bit, now how many bit we need to borrow that we can not find from given information but it should be atleast 3 bit bcz we know that DBA contain all host bit 1.
x.x.x.31
.31=.00011111
now 3 bit of last octet can be used as subnet(e.i 000 means 0th subnet) ==> subnet mask .224
like 4 bit of last octet  can be used as subnet(e.i 0001 means 1st subnet)==> subnet mask .240
like  5 bit of last octet  can be used as subnet(e.i 00011 means 3rd subnet)==> subnet mask .248
and so on.
0th subnet also known as net mask and netmask pahle kisi bhi particular network ko define karne ke liye use karte the, it is obsolete now, ab as a address bhi use hone laga h so both .240 and 224 are valid in different circumstances.

Answer 2

All 255.255.255.224,255.255.255.240,255.255.255.248,255.255.255.252,255.255.255.254 are correct Direct broadcast address for given question X.X.X.31 as it has 31 as broadcast so it can have last octet subnet address as :- 0 (255.255.255.224),16(255.255.255.240),24(255.255.255.248),28(255.255.255.252),30 (255.255.255.254) but only in given option with maximum 1's it is option A.Even your answer  255.255.255.224 is correct as well as option A.I think its ambiguous question.