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# Subnetting Help

I am currently reading/learning about subnetting I have mostly got the hang of it now but am struggling with
a couple of questions on the practice exam.

what is the valid ip range for the subnet 144.45.24.0/21?

what is the valid ip range for the subnet 140.10.10.0/23?

the number of valid subnets on the 20.0.0.0  255.254.0.0 is ?

please could someone explain the process to work out the above I'm fine with anything /24 or above but
stuggling with these.

Many Thanks
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Ross-C
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2 Solutions

Commented:
144.45.24.0/21

Valid Host IP Range 144.45.24.1 - 144.45.24.254

140.10.10.0/23

Valid Host IP Range 140.10.10.1 - 140.10.11.254

20.0.0.0  subnet 255.254.0.0

20.0.0.0/15
Valid Host IP Range 20.0.0.1 - 20.1.255.254

The rational behind the slash notation "/" is the number of bits of the binary representation of the IP address are used to determine the subnet address.
"/24" is easy because it uses all of the first three octets to determine the subnet leaving you with just the last octet for hosts, x.x.x.1 - x.x.x.254

i.e. 255.255.255.255 = 11111111.11111111.11111111.11111111
so a "/15" subnet means that the first 15 bits are the network address and the rest is used for determining host addresses.
so for the 20.0.0.0 example:
"00010100.0000000"0.00000000.00000000 = 20.0.0.0/15 the quoted portion is the bits used for the subnet.
So the valid host IP range will be from 0.00000000.00000000  to 1.11111111.11111111
convert from binary and you get x.0.0.1 to x.1.255.254 (excluding the all 0s and all 1s as those addresses are used for the network address and broadcast address)
The first 15 bits being the subnet gives you the subnet mask:11111111.11111110.00000000.00000000 = 255.254.0.0

This is a really abbreviated explanation and relies on an understanding of how to count in binary, or at least convert from binary to decimal and back again.

Hope this helps. You can do a search online for Subnet calculators that will take the work out of calculating the subnet masks and valid hosts for a giving ip/#
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Commented:
I get slightly different answers for the first:

what is the valid ip range for the subnet 144.45.24.0/21?
144.45.24.1 - 144.45.31.254

and third:
the number of valid subnets on the 20.0.0.0  255.254.0.0 is ?
254, or if it's old school which doesn't allow an all zero subnet, 253

@Ross-C, for the above I assume you understand the underlying binary and just want an easy way to work it out?
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Author Commented:
Yes that's right Frabble. I have been studying submitting and understand the binary etc. I am just struggling with the best way to work out questions like the ones in my post.
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Commented:
Thanks for the correction Frabble. That was a typo on my part for the 144.45.24.0/21 address range.
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Author Commented:
Many thanks to you both
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