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Subnetting Shortcuts

 

Class C

 

200.4.5.0       Borrow 3 bits           Subnet Mask = 255.255.255.224

Number of networks created 23 = 8
Number of useable networks created 23 = 8 – 2 = 6

Number of host per network 25 = 32
Number of useable host per network 25 = 32 – 2 = 30

The increment for each network is 32

 

If you need to determine the network number of subnet 6,
multiply 6 X 32 = 192.
The subnet 6 network address would be 200.4.5.192

 

In simple terms, multiply the increment by the number of the subnet
you wish to find.

 

Class B – borrowing in the 3rd octet

 

150.4.0.0       Borrow 3 bits           Subnet Mask = 255.255.224.0

Number of networks created 23 = 8
Number of useable networks created 23 = 8 – 2 = 6

Number of host per network 2X 256 = 8192 

The increment for each network is 32 in the 3rd octet
(the number of unmasked bits in the 3rd octet is 5, this is
the octet we borrowed from)

 

If you need to determine the network number of subnet 6,
multiply 6 X 32 = 192.
The subnet 6 network address would be 150.4.192.0

 

If you are borrowing in the 3rd octet, just ignore the 4th
octet to determine your network numbers.

 

Class B – borrowing all the 3rd octet

 

150.4.0.0       Borrow 8 bits           Subnet Mask = 255.255.255.0

Number of networks created 28 = 256
Number of useable networks created 28 = 256 – 2 = 254

Number of host per network 2X 256 = 256 

The increment for each network is 256 in the 3rd octet.
(The number of unmasked bits in the 3rd octet is 0, this is
the octet we borrowed from.
This means that the value of the 3rd octet increases by one each time.)

 

If you need to determine the network number of subnet 26, simply
insert that number into the 3rd octet slot.
The subnet 26 network address would be 150.4.26.0

 

If you need to determine the network number of subnet 100, simply
insert that number into the 3rd octet slot.
The subnet 100 network address would be 150.4.100.0

 

Class B – borrowing in the 4th octet

 

150.4.0.0       Borrow 11 bits                   Subnet Mask = 255.255.255.224

Number of networks created 211 = 2048  (or 23 X 256 = 2048)    

Number of useable networks created 211 = 2048 – 2 = 2046

Number of host per network 25 = 32 

The increment for each network is 32 in the 4th octet.
(The number of masked bits in the 4th octet is 3, the number of
unmasked bits in the 4th octet is 5, this is the last octet we borrowed from.
This means that the value of the 4th octet increases by the increment value.)

 

If you need to determine the network number of subnet 325, do the following:

 

Divide the desired subnet by 2 raised to the masked bits in the 4th octet         

325 / 23    or  325 / 8 =  40 remainder 5

The 40 is the value of the 3rd octet  --  150.4.40.?

To find the value of the 4th octet multiply the remainder (5) times the increment (32)

32 X 5 = 160    This is the value of the 4th octet

 

The subnet 325 network address would be   150.4.40.160

--------------------------------------------------------------------------------
 

If you need to determine the network number of subnet 40, do the following:

 

Divide the desired subnet by 2 raised to the masked bits in the 4th octet         

40 / 23    or  40 / 8 =  5 remainder 0

The 5 is the value of the 3rd octet  --  150.4.5.?

To find the value of the 4th octet multiply the remainder (0) times the increment (32)

32 X 0 = 0    This is the value of the 4th octet

 

The subnet 40 network address would be   150.4.5.0

 


Another Method

Class B – borrowing in the 4th octet

 

150.4.0.0       Borrow 11 bits                   Subnet Mask = 255.255.255.224

Number of networks created 211 = 2048  (or 23 X 256 = 2048)    

Number of useable networks created 211 = 2048 – 2 = 2046

Number of host per network 25 = 32 

The increment for each network is 32 in the 4th octet. (The number of
masked bits in the 4th octet is 3, the number of unmasked bits in the
4th octet is 5, this is the last octet we borrowed from.
This means that the value of the 4th octet increases by the increment value.)

 

Another way to determine the Network Number of a Subnet:

 




   
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