Subnet Masking

While applying a subnet mask to the existing IP address you have to identify the the network and the node part of the address.

The network bits will be represented by the 1s in the mask , and the node part is represented by the 0s.

Performing a bitwise logical AND operation between the IP address and the subnet mask results in the Network Address or Number.

For example, using our test IP address and the default Class B subnet mask, we get:
 

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10001100.10110011.11110000.11001000 140.179.240.200 Class B IP Address
 
11111111.11111111.00000000.00000000 255.255.000.000 Default Class B Subnet Mask
 
10001100.10110011.00000000.00000000 140.179.000.000 Network Address

10001100.10110011.11110000.11001000 140.179.240.200 Class B IP Address

11111111.11111111.00000000.00000000 255.255.000.000 Default Class B Subnet Mask

10001100.10110011.00000000.00000000 140.179.000.000 Network Address

Default subnet masks:

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Class A - 255.0.0.0 - 11111111.00000000.00000000.00000000
 
Class B - 255.255.0.0 - 11111111.11111111.00000000.00000000
 
Class C - 255.255.255.0 - 11111111.11111111.11111111.00000000
 
CIDR -- Classless InterDomain Routing.

Class A - 255.0.0.0 - 11111111.00000000.00000000.00000000

Class B - 255.255.0.0 - 11111111.11111111.00000000.00000000

Class C - 255.255.255.0 - 11111111.11111111.11111111.00000000

CIDR -- Classless InterDomain Routing.

CIDR was invented several years ago to keep the internet from running out of IP addresses. The "classful" system of allocating IP addresses can be very wasteful; anyone who could reasonably show a need for more that 254 host addresses was given a Class B address block of 65533 host addresses.

Even more wasteful were companies and organizations that were allocated Class A address blocks, which contain over 16 Million host addresses! Only a tiny percentage of the allocated Class A and Class B address space has ever been actually assigned to a host computer on the Internet.

It is expected that CIDR will keep the Internet happily in IP addresses for the next few years at least.

After that, IPv6, with 128 bit addresses, will be needed. Under IPv6, even sloppy address allocation would comfortably allow a billion unique IP addresses for every person on earth