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Subnetting question

N/A

Subnetting question

800 clients, 40 Servers.
You have been told that 10.20.32.0/20 are availible for your use.
The idea is to use a mask/addressing scheme that is as close to the requirements as possible.

The answer states you should use a mask of /22 or 255.255.252.0, start IP 10.20.32.41 end ip 10.20.35.254.
Why the jump from 32 to 35?

Thanks
4 REPLIES
pacem
Grafter
Posts: 175
Registered: 07-09-2007

Subnetting question

because there are 976 ip addresses in the range 10.20.32.41 to 10.20.35.254 which is plenty for your 800 client machines. Personally I would have reserved a few spare addresses for the servers (I assume they're ...32.1 to ...32.41) since the netmask allows 1024 ( -8 ) IP addresses in total.

...you do know each number in the IP address is in the range 1 - 254 ?

Paul.<><
N/A

Subnetting question

Hi,

Thanks for your reply. Can you break down the answer any further? Shockedops:

Thanks for your help.
pacem
Grafter
Posts: 175
Registered: 07-09-2007

Subnetting question

OK,
first look up how binary numbers work.

Good.

Now your IP address is made up of 4 8-bit binary numbers. As an 8-bit binary number can represent the decimal values from 0 to 255 each dotted number in your IP address can only be between 0 and 255.

You require 800 PC IP addresses + 40 Servers so that is 840 addresses.

Now going back to binary, with 8-bits you can count up to 255, with 9 up to 511, and with 10 bits you can count up to 1023 which is just more than you need so we need 10 bits to count all of your IP addresses.

In an IP address there are 4 binary numbers each with 8 bits the whole address is 32 bits long.
You need 10 bits to count from 0 to 1043 for your servers so that leave 22 bits that don't change. This 22 is your mask. (count the 1s)
11111111.11111111.11111100.00000000
10.20.32.0 in binary is:
00001010.00010100.00100000.00000000
The full 1023 address range would be from the above to:
00001010.00010100.00100011.11111111
10.20.35.255
Note the last 10 bits are now 1s.
Now in IP addressing 0s and 255s are special so your servers use 10.20.32.1 (and not 0) to 10.20.32.41, and your clients then use ...42, ...43 etc up to 10.20.32.254 then after that (10.20.32.255 is special and isn't used) there is no room for more 1s in the last 8 bits (we can't represent 256 without an extra 1) so we increment the 9th bit from the right and start again with the last 8 bits (skipping 0) so 10.20.33.1 ...2 ...3 ... ...254, 10.20.34.1 ...2 etc up to 10.20.35.254 which would actually give you a few too many addresses.

I hope that is the sort of answer you were looking for even if it's not the clearest of answers.

Paul.<><
N/A

Subnetting question

Paul,

Thanks for your answer, it is appreciated and a lot clearer Cheesy

Subnetting is one topic i need to work on for MCSA!

Simon