ISRO-DEC2017-31

Question

In the IPv4 addressing format, the number of networks allowed under Class C addresses is

• A. $2^{20}$
• B. $2^{24}$
• C. $2^{14}$
• D. $2^{21}$

Comments

Every IPv4 is a 32 bit representation.

Below is the case for CLASSFUL IP ADDRESSES

In Class-C first 24 bits are used for NetworkID part and last 8 bits are used to identify host id.

So we think 2^ 24 networks possible but it is not correct because in class-C first three bits are always 110.

so remaning bits for NID part is 24-3=21

so with 21 bits we can form 2^21 different networks.

Just like this in Class-A first bit is always (0) Therefore number of networks in class-A is 2^7

Just like this in Class-B first two bits are always (10) Therefore number of networks in class-A is 2^14

---

can anyone tell me for class D and class E.

Answer 1

Every IPv4 is a 32 bit representation.

Below is the case for CLASSFUL IP ADDRESSES

In Class-C first 24 bits are used for NetworkID part and last 8 bits are used to identify host id.

So we think 2^ 24 networks possible but it is not correct because in class-C first three bits are always 110.

so remaning bits for NID part is 24-3=21

so with 21 bits we can form 2^21 different networks.

Just like this in Class-A first bit is always (0) Therefore number of networks in class-A is 2^7

Just like this in Class-B first two bits are always (10) Therefore number of networks in class-A is 2^14

Answer 2

Class C = 24 network bits + 8 host bits.

But first 3 bits (110) are fixed. Hence only 2 ²¹ networks are possible.

Answer is D

Answer 3

Class C has three octets in NID, so 24 bits.

Out of which three initial bits are fixed to 110.

So, available bits = 24 - 3 = 21.

Option D