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subnetting subnet masks - urgent !

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Hey, i am pretty new here, i have a question about subnetting and subnet masks, i would appreciate if u could answer me soon, it is very urgent, thank u :) and excuse my language i will have to translate the question:

A company has a C class address: and wants to create 4 subnets

while department A has 72 computers, B has 35,Chas 20 and D has 18 computers = total 145 computers

a) what are the subnet masks to make this process possible?

b)what should the company do if department D increases its computers to 34?

i have been browsing for hours but whatever i do i come to a conclusion that i can manage 2 exponent 6 = 64 -2 = 62 Ip hosts every time, i cant figure out how to let 72 computers in a submask under these circumtances and also i cant see the point of the (b) part...

i hope u could help me with solving this problem, thx :)
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Nihonjin desu ka?

All you really need is a good IP Subnetting tutorial book.

It sounds like you are taking a Cisco CCNA test. The trick to the above question is to know you cannot come up with a subnet mask for exactly 72 IP address or 35, 20, etc. Class C subnetting works like this. ( I will not get heavy into the binary math. Binary math is in every subnetting book in the world. Just the facts.) with subnet mask of = one network with 256 IP addresses. Subtract an address for the network ID ( and the broadcast ID. (

If you borrow one bit from the last octet to change the subnet mask to then this =
2 possible networks with 128 ip addresses each. Each network will have 126 "usable" addresses because of the network ID and broadcast.
(1st and last IP address in each range.) - is the first network. - is the second network.

Next subnet - = 4 networks with 64 IP addresses each. (don't forget to subtract 2) - 1st network. - 2nd network - 3rd network - 4th network = 8 networks with 32 ip addresses each. = 16 networks with 16 ip addresses each. = 32 networks with 8 ip addresses each.

Variable Length Subnet Mask, (VLSM) means your router will recognize subnetted networks of different sizes.
To solve your problem you need to use a subnet with 128 ip addresses to meet your 72 ip address requirement.
You need to use a subnet with 64 ip addresses to meet your 35 ip address requirement.
You need to use a subnet with 32 ip addresses to meet your 20 ip address requirement.
You need to use a subnet with 32 ip addresses to meet your 18 ip address requirement.
128+64+32+32 = 256. (That's all you have.)

DON'T overlap any of the networks.

If D increases to a requirement for 34 ip addresses you need another class C network.

I know this is a 100% perfect explaination, but as I said, there are about 100 different good subnetting books on the market.

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