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For a class C network if IP address of a computer is 200.99.39.112 and subnet mask is 255.255.255.224 the decimal value of last octet of last host of sixth subnet is ?

shouldn't it be 190 but the answer given is 222? anyone help

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6 votes
6 votes

class C default NetWork mask : 255.255.255.0 ..But subnet mask is 255.255.255.224

224 = 11100000 = 128 + 64 + 32

so, 3 1's have been borrowed from host bits which will now become subnet bit.  

for subnets there are 8 subnets ( 23 = 8 ) possible like 000, 001, 010 , 011 and so on .....  the first subnet is 000, second subnet 001, third 010 and fourth 011, fifth 100 , sixth 101 , seventh 110 , eight subnet 111 .

Hence subnet no of 6th subnet = 101 , 

The address with all 1s ( 111 11111 ) in host part is broadcast address and can't be assigned to a host. So the maximum possible last octal in a host IP is 111 11110 , 

here it asked 6th subnet so last octet of last host will be 101 11110 .

The decimal value of last octet of last host of sixth subnet is 101 11110 = 128+32+16+8+4+2 =190.

edited by
3 votes
3 votes
last address of sixth subnet will be 209.99.39.11011110   last octet 222
3 votes
3 votes

If we divide according to decimal value then 110 part subnet otherwise 101 so according to 110 - 222(ans), 101 - 190(ans).

1 votes
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As the given IP belongs to network of class C we know that out of 32 bits the first 24 bits are reserved for the nid . The subnet mask given to us is 255.255.255.224.

The last octet in this gives us the information about the subnet id bits i.e 3 as 224=11100000

So they are asking for the decimal value of the last octet of the last host of the 6th subnet ... So SID =110 As it is the last host rest all bits are 11110(last bit is not one because if it was 1 then it becomes directed broadcast address)

So we get 1101 1110=128+64+16+8+4+2=222.

Note: They are referring to subnet with id =6 if 6th subnet would be there then id would be 101 and ans will be 190 as

000- 1st subnet 001=2nd subnet 010- 3rd subnet 011-4th subnet 100-5th subnet 101-6th subnet 110-7th subnet 111-8thsubnet
Answer:

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