CIDR Notation

•Inside the computer each address mask is stored as a 32 bit value in binary, which is then expressed in dotted octet notation.

•The new CIDR notation append a slash and the size of the mask in decimal notation:

       For example 128.10.0.0/16

CIDR Address Block Example

•Suppose an ISP has a single Class B license 128.211.00.0.  Using a classful address scheme, he/she can only assign the prefix to one customer, who can have up to 216 host addresses.

•Using CIDR, the ISP could assign the entire prefix to a single organization by using 128.211.0.0/16

•Or he could partition the address into three pieces (two of them big enough for 2 customers with 12 computers each and the remainder available for future use.

••One customer could be assigned 128.211.0.16/28

•and the other could be assigned  128.211.0.32/28

•Both customers have the same mask size (28 bits), but the prefixes differ and each has a unique prefix.  More importantly the ISP retains most of the addresses, which can then be assigned to other customers.

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