Consider a classless IP address 173.91.43.43/25. Which of the following is/are correct about the number of IP addresses, first IP address, the last and valid host IP address in 173.91.43.43/25 IP address?a)Number of IP addresses: 127b)First IP address: 173.91.43.1c)Last IP address: 173.91.43.127d)Valid Host IP address: 173.91.43.64Correct answer is option 'C,D'. Can you explain this answer?

Question

Answer

The given IP address is 173.91.43.43/25. Let's analyze the different aspects of this IP address.

Number of IP addresses:
The /25 in the IP address indicates the subnet mask. In this case, the subnet mask is 25 bits long. To calculate the number of IP addresses, we subtract the subnet mask length from 32 (the total number of bits in an IPv4 address) and take 2 to the power of the resulting value.

32 - 25 = 7
2^7 = 128

Therefore, the number of IP addresses in the given subnet is 128.

First IP address:
The first IP address in a subnet is always the network address. To calculate the network address, we take the given IP address and set all the host bits to 0. In this case, the host bits are the last 7 bits since the subnet mask is /25.

173.91.43.43 => 10101101.01011011.00101011.00101011
Setting the last 7 bits to 0: 10101101.01011011.00101011.00000000

Converting back to decimal: 173.91.43.0

Therefore, the first IP address in the given subnet is 173.91.43.0.

Last and valid host IP address:
The last IP address in a subnet is always the broadcast address. To calculate the broadcast address, we take the network address and set all the host bits to 1. In this case, the host bits are the last 7 bits.

Network address: 173.91.43.0
Setting the last 7 bits to 1: 10101101.01011011.00101011.01111111

Converting back to decimal: 173.91.43.127

Therefore, the last and valid host IP address in the given subnet is 173.91.43.127.

In summary:
- Number of IP addresses: 128
- First IP address: 173.91.43.0
- Last and valid host IP address: 173.91.43.127

Answer

IP address: 173.91.43.43./25

Given,

Number of mask bits = n = 25

In classless addressing, no. of address is given by 2^{32 − n}.

No. of addresses = 2^{32 − 25}= 2⁷ = 128

Therefore option (A) is incorrect.

For first address set 32 − n rightmost bits to zero.

i.e. set 7 rightmost bits to zero.

43 → 00101011

First address → 173.91.43.0

Therefore, option (B) is incorrect.

For last address set 32 − n rightmost bits to 1. i.e. set 7 rightmost bits to 1.

43 → 00101011

last address → 173.91.43.127

Therefore, option (C) is correct.

Range: 173.91.43.0 and 173.91.43.127

First and Last IP address are excluded,

173.91.43.64 is valid host IP address.

Therefore, option (D) is correct.

Hence, the correct options are (C) and (D).

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