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Subnetting questions

Im taking some CCNA practice exams. The ONLY part thats giving me trouble are these two types of subnetting questions:

1. Enter the number of networking bits that would give a network with 2 host addresses.

I know the answer, but I dont know how I got it lol
its 255.255.255.252
or 30bits. I only know that from subnetting in real environments. How do you properly answer this question?

2. Enter the subnet mask dotted decimal value that would give a network with 2,046 host addresses

ASKER CERTIFIED SOLUTION

minmei

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ASKER

q1: is there a formula for this? I noticed they said number of hosts and not host bits. Im only used to questions that ask something in bits etc.

q2 is actually trial and error? You play with 2^x-2=your answer
Thats what I figured... I guess you just try all types of combos with questions like this.

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-Leo-

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ASKER

Leo, thanks. Where did you get the numbers "3=11" in questions 1?

-Leo-

3 will be 11 in binary
2 will be 10 in binary
255 wil be 11111111 in binary

You know the joke: there is 10 types of people in the world: who understand the binary and who is not :)

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ASKER

oh 3 is 00000011
2 is 00000010

Gotcha. thanks man

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ASKER

so if you want 510 hosts, you would need 16 host bits?

255.255.0.0

1111111.11111111.00000000.00000000

Is this correct?

minmei

Nope.

510 and add 2 (network and broadcast) = 512

512 = 2 to the 9th power = 9 bits for hosts

11111111.11111111.11111110.00000000 - hosts are zeros

subnet mask is

255.255.254.0

Do the math for 30 hosts. I'll check you.

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ASKER

Wait I think I see. When ever they ask:
"enter the number of networking bits that would give a network with 1022 hosts"

The first thing you do is add +2 to 1022. This is because one is reserved for network, the other reserved for broadcast. This gives us 1024.

Now, I just have to do the formula 2^x=y to find the 1024.
2*2*2*2*2*2*2*2*2*2*2 =1024
Thats eleven "2s", or eleven host bits

11111111.11111111.11111000.00000000
255 255 248 0

eh, is this right?

minmei

Close.

2 to the 10th power is 1024, not eleventh. So what would the mask be now?

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ASKER

11111111.11111111.11111100.00000000
255.255.252.0

22 networking bits 10 host bits

Is this correct?

minmei

Absolutely.

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ASKER

thanks! finally the light has come on. Let's hope it doesnt go out LOL