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# how to culculatye no of computer

Posted on 2004-11-20
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Last Modified: 2010-04-10
255.255.25525.2 =3 comuter
255.255.255.0     = 254 computers

How to culculate it

What is the subnetmark
0
Question by:teera
3 Comments

LVL 3

Assisted Solution

Hello

I'm not sure what you mean by that, but there is an excellent
calculator for ips and subnet masks - here is the link to it - its free and its online -

http://www.telusplanet.net/public/sparkman/netcalc.htm
0

LVL 15

Assisted Solution

Here's how I do it - you use the last non-zero octet of the mask and subtract it from 256. This is used to determine the number of IP addresses (based on which octet) and then you need to subtract 2 (for the all zeros network address and all ones broadcast address) for the number of available addresses.

Examples:
Subnet mask 255.255.255.252
Last non zero octet is the fourth, 252. Subtract this from 256 to get 4. Subtract 2 to give 2 "computers" (not 3)

Subnet mask 255.255.255.224
Subtract 224 from 256 to give 32, minus 2 to give 30 available addresses

Subnet mask 255.255.255.0
Subtract third octet value 255 from 256 to give 1. Since this is the third octet, you need to multiply by 256, which gives 256. Subtract 2 to give 254 available addresses.
If it's the second octet, you multiply by 65536 (256 x 256) and the first is multiplied by 256 again - you probably will need a calculator for these.

Last one - subnet mask 255.255.248.0
248 from 256 gives 8, times 256 is 2048, minus 2 is 2046 available addresses.

0

LVL 1

Accepted Solution

If you want to go down to the bit level, its a bit more complicated, but you might be able to understand it a bit more...

lets say subnetmask of 255.255.255.252

128    64      32      16      8      4      2       1
last octet is =    1       1       1        1      1       1      0       0
so therefore.. you have the first 6 bits for subnets, and the last 2 bits for hosts..

as always, all 0's in the host portion, is the network address, and all 1's in the host portion is the broadcast address....
so that leaves 2 address's for hosts...  this subnetmask would only be used usually for a portion between routers or of the kind, because of its limited host addresses

try another example  ...   255.255.240.0

5096   2048  1024  512   256
3rd octet                                                                              4th Octet
128    64      32      16      8      4      2       1       |    128     64      32      16      8      4     2     1
third octet is =   1       1       1        1       0      0      0       0       |      0        0        0       0       0      0     0     0

so you have 4 bits in this octet for the network portion (1's) , and 4 for the host portion (0)'s (but remember this is the third octet, so have another 8 bits for the hosts..)

so, you have 254 for the hosts from fourth octet before even counting the third octet...  then what i do is just keep increasing the bit value from the 4th octet to the third.. so 128, 256, 512, 1024, 2048, and 5096  now 5096 is the one to look at....   1 less than that is the number of hosts you have in the host portion... take away 2 for the network address and the broadcast address, and you have 5094

so with subnet mask of 255.255.240.0 you have 5096 hosts available..  and the rest are subnet addresses (15 addresses - 13 of those usable)
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