IP

Question

If IP is 210.15.131.141 and Subnet Mask is 255.255.255.224 Then what is the IP of last host of last subnet?

Answer 1

IP-:$210.15.131.141$

Mask-:$255.255.255.224$

$\Rightarrow$Seeing the IP we can guess that it is class C$\Rightarrow$Network bit=24 bit and host=8 bit

$\Rightarrow$Its mask must be $11111111.11111111.11111111.00000000$ but it is actually $11111111.11111111.11111111.11100000$$\Rightarrow 3$ bit is taken away from host $\Rightarrow$3 bit must be its subnet  

$\Rightarrow$ subnet bits are $000$,$001$,$010$,$011$,$100$,$101$,$110$,$111$

$\Rightarrow$ ANDing IP with mask we get Network address of that IP

        $11010010.00001111.10000011.10001101$

AND

        $11111111.11111111.11111111.11100000$

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$11010010.00001111.10000011.10000000$

Also,last subnet is $111$

So Finally addrress of last host of last subnet is $11010010.00001111.10000011.10000000.111111110$

210.15.131.254

Comments

How is the Binary equivalent of Mask 255.255.255.224 is 11111111.11111111.11111111.11000000(sum of first 2 bits in the last octet is 192)
It should be 11111111.11111111.11111111.11000000(first 3 bits in the last octet should be one right as it is 224).
Also can you please elaborate the last step where you used the subnet to calculate the last host address.

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updated !thanks!

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111 cant be alloted for subnet. 111 is used for broadcasting to all subnets.The last octet of the address must be 11011110

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@Sushant Gokhale why?

Dba is netid+all 1's in host I'd.

why we can't use subnet 110?

Then you are saying that with 3 bits we can't have 3subnet?