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Since organistation has 30 hosts, Class C network with /24 Prefix should be used since it can support 254 hosts.

Now there are 30 hosts, so the minimum number of bits required for host number is 5, since $2^5 - 2$ is 30.

The rest of bits i.e 3 can be used as subnet number.

So the mask is 11111111.11111111.11111111.11100000

For reference have a look at http://andthatsjazz.org/wbglinks/pages/reads/misc/ip.html
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y  this ??????????Since organistation has 30 hosts, Class C network with /24 Prefix should be used since it can support 254 hosts.

wt u want to say y 254 ???

+1
In class C network first 24bits are network bit and last 8 bit are host bit.. with 8 bit total 256 host but two notation (all zros and all ones)reserved for special purpose .. so total 256 -2 = 254 host possible..
Out of c those 254 combination we need 30 combination.. i.e last 5bits are sufficient..
+2

Their are two types of classifications . As you know class A class b c d e ... and another is classless . 

the advantage of class less is we can distribute no. of ip according to their demands ( Should be power of 2 ) . according to you if /24 happens means the organization have to buy 2^8 means 256 ip but only 30 ip is needed .226-2 = 224 ip (one for netid and one for broadcast add which we cant use as host id ) will be unused , for that reason classless comes in to the picture , /5 for this organization buys only 32 ip ,  see the wastage is reduced .  so the mask will be 11111111.11111111.11111111.11100000



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