ok
thanks for the complement that my answers are easy to 
understandble

now comes on topic subnating
brekaing a single network in to the multiple subnetwork  is 
called subnating
sub+nat+doing=subnating(trick for rember the diffination)
why we need it-
in that decade where all the industry and corporte are 
being comptrized but for the comunicatin on the n/w channel 
it is nessry that all ip address should be unique so 
maintain the uniquecity of that ip address subnating is 
used 
this is the technical explanation if some one say that 
scince ip addres is not enough so to preserve the ip 
address we did subntaing this is also wright but not a good 
approch
this is all about theroytical consepts of sunating
now how we done it

here it is(i am reqesting you to write it some where becoz 
it is the back bone of networking) 

now as you know that threr are classes of ip address

as class a    range of class a 1---126  mask 255.0.0.0

   class b    range of class b 128--191 amsk 255.255.0.0
         
   class c   range of class c  192--223 mask 255.255.255.0

   class d    range of class d  224-255 mask 255.255.255.255

   class e       range of class e 
here some point you should remebr
1  in class a i didnot define 0 and 127 because 0 is resved 
for default root (you will read it in routing protocol)amd 
127 is reseved for loop back(familar with 127.0.0.1 for 
checking nic card stautush wright if not then try it to cmd 
promt )
2 network 224 - 239 resved for multicasting and class e 240-
255 for some defence uses
so not be confused becoz network 224 to 255 are reseved for 
class d and class e addresss

now let start subnating with class c adress

class c address
 in that adress 3 octet are for the networking and 1 octet 
for host
that is    
class c        N|N|N|H
each n/w bit represnted by N
and host bit by H(plz keep in mind/i will use it in that 
whole article)
ok each network bit coantin 8 bit
and each host bit conatin 8 bit
then if threr is ip asrees 192.168.10.12 then what should 
be mask
answer is 24
now question is what is mask
mask is used to identyfing that from which class ip address
so come to th point
ip address=192.168.10.12/24
now why 24?
becoz in class c ip address there are 3 octet for network 
of size 8 bit then 8*3=24.so what did you notice that for 
identyfing the ip address class we see the only value of 
first octet only first (remeber)

so in the subnating we answered folowing question
1 how many subnet does the chosen subnet mask produse
2 how many vallid host per sunet are avilable
3 what are the brodcast address of each subnet
4 whta are the valid host in each sunet
5 what are the valid sunet

befor doing sunating these two equation must be rembred
that is 
2^n-2=for total nuber of network   n=network bit
2^h-2=for total number of host bit h=avilable host bit
why -2(becoz cisco not recomnded it as a valid n/w and host)

let understand by an exapmle
192.168.10.0/26

ok what should be original mask for that address 
answer is
192.168.10.0/24 but given mask is 26 that is we need 2 bit 
extera from where we get it
we get it from hot bit becoz it has 8 bit for host
now we borrow 2 bit from host then remnaig bit are 6
so how many n/w soory vallid network =2^2-2=2
and what is the nimber of valid host=2^6-2=62 host per sunet
now what should be mask of that given ip address
herer it is scince we borrow two bit from host for n/w then
128 64 32 16 8 4 2 1 (what is this?=this is some thing to 
convert any decimal value to binary in eifght add all u get 
255)
again i wrote 128  64  32  16  8  4  2  1
                1   1    0   0  0   0 0  0 
two bit is taking for sunating so i amrk two bit as one and 
rest are as 0
so add this two bit marken by 1
128+64=192
then mask of that given ip will be
255.255.255.192/26(see that mask i decriebed)
 so what should be block size
tha is 256-192=64
that is subnated ip address is
192.168.10.0    192.168.10.63   192.168.10.127   10.191
-----------     ------------    ----------       ------
-----------     ------------    ---------        ------
192.168.10.63    192.168.10.126  192.168.10.190  10.255

what is n/w id=first ip address is always known as n/w id
 what is broadcast id=last ip address is broad cast id
for given ip 192.168.10.12/26 what is the n/w id and broad 
cast id
answer os n/w id 192.168.10.0
broad cast id  192.168.10.63
simalry for192.168.10.92 what is the n/w id and broadcast 
id 
answer is 192.168.10.64(n/w id)
          192.168.10.126(brad cstid)
ok here cisco let the invalid ip the first one and last one 
that is
192.168.10.0---------192.168.10.63
 192.168.10.191  -----192.168.10.255
valid host
the ip address between the n/w id and host id
so herer is your task
in need reply
wsunat that ip address
192.168.56.64/27
and answer all short of thing

after that i will go to next section

Hi Jatendra thanks for such a nice tutorial.....

but i feel a little confusion in this step

192.168.10.0    192.168.10.63   192.168.10.127   10.191
-----------     ------------    ----------       ------
-----------     ------------    ---------        ------
192.168.10.63    192.168.10.126  192.168.10.190  10.255
 
In this how take start from ..10.63 and after that increment
64 each time. my point is 63. how we know that the first
address of subnet will start from .63 .plzz clear this one

hi arshad 
here is expalnation 
i have mistaly write next sunet id that is
192.168.10.0    192.168.10.63****( my mistake it start 
from                               (192.168.10.64)  
-----------     ------------    
-----------     ------------    
192.168.10.63    192.168.10.126 

if this is your point of confusion then i ma soory i donot 
know how i have writenn it.

now how to detrmine the exact block size and what will be 
next sunet id(it also called n/w id)
then i am going to explain again
what was the given ip address 
192.168.10.0/26
ok 2 bit of subnating i.e two bit is taken from host bit
i.e 

128 64 32 16 8 4 2 1
1    1  0  0  0 0 0 0
tha is two bit is taken from host
now how many n/w will be made 2^2-2=2(acording to cosco 
nither 4)
how many bit for host it is 6
numer of host / sunet=2^6-2=62
what is the mask of that given ip
255.255.255.192
what is the block size=256-192=64******************
block size consern with how many ip address should be 
reside in the single n/w pool
that is therer is 64 ip address reside in the pool.now if 
we started it with 0   than 64th ip address is end with 63
count 0 to 63 how many bit is it 64 if not then count again
(kidding)ok
so one pool is end with 63 then next pool with started at 
what is it 64? yeas and go the 127 again 64 host
now any confusion?
the try to solve that question which i have provide and 
there is another probem i am providing try to solve i am 
90% sure you canot but try?


what is the n/w id and the brodcast id for the ip address 
192.168.10.56/27

try it then i wiil send the tutreala about vlsm design in 
claas c

scince first one is n/w id the last one is broadcast id 
then vallid host ip address is 62(see the formula)