Classless Inter-Domain Routing CIDR

Classless Inter-Domain Routing (CIDR)

1.Classless Inter-Domain Routing (CIDR) is a methodology of allocating IP addresses of same class into one single address to reduce the size of routing table.
2.Subnet mask can be represented as Classless Inter-Domain Routing (CIDR)
3.CIDR is done on core router to reduce the size of routing table.
4.CIDR is implemented by ISP(Internet Service Provider).
5.No concept of Class A, B, C networks.


Suppose 192.168.10.45/29 is an IP address. 29 means, there will be twenty-nine 1s at the beginning followed by three 0s:
11111111 11111111 11111111 11111000
That means we have a block that contains 8 addresses which means that the 3 bits in the address are for the host and the remaining 29 bits are the network.
The beginning address in each block must be divisible by the number of addresses in the block. A block that contains 8 addresses can not have beginning address as 192.168.1.9
But the address 192.168.1.16 is possible. This 16 can be the starting address.

The CIDR notation 10.10.0.0/8 would be written as 10.10.0.0/255.0.0.0
The CIDR notation 172.16.0.0/16 would be written as 172.16.0.0/255.255.0.0
The CIDR notation 192.168.0.0/24 would be written as 192.168.0.0/255.255.255.0

•If two bits were borrowed, the mask for a Class C address would be 255.255.255.192 or 255.255.255.192=/26
•The fourth octet, 192 = 11000000

128	64	32	16	8	4	2	1
1	1	0	0	0	0	0	0

128+64=192

The /27 = the total network and subnet bits.
For Class C network, this means 3 bits are borrowed;
255.255.255.224 = 255.255.255.11100000
For  Class B network, this means 11 bits are borrowed;
255.255.255.224 = 255.255.11111111.11100000
For Class A network, this means 19 bits are borrowed;
255.255.255.224 = 255.11111111.11111111.111000000


The /26 means that :
For Class C network, this means 2 bits are borrowed;
255.255.255.192 = 255.255.255.11000000
For Class B network, this means 10 bits are borrowed;
255.255.255.192 = 255.255.11111111.11000000
For Class A network, this means 18 bits are borrowed;
255.255.255.192 = 255.11111111.11111111.11000000

Class A

CIDR	host bits	Netmask
/8	24	255.0.0.0
/9	23	255.128.0.0
/10	22	255.192.0.0
/11	21	255.224.0.0
/12	20	255.240.0.0
/13	19	255.248.0.0
/14	18	255.252.0.0
/15	17	255.254.0.0

Class B

CIDR	host bits	Netmask
/16	16	255.255.0.0
/17	15	255.255.128.0
/18	14	255.255.192.0
/19	13	255.255.224.0
/20	12	255.255.240.0
/21	11	255.255.248.0
/22	10	255.255.252.0
/23	9	255.255.254.0

Class C

CIDR	host bits	Netmask
/24	8	255.255.255.0
/25	7	255.255.255.128
/26	6	255.255.255.192
/27	5	255.255.255.224
/28	4	255.255.255.240
/29	3	255.255.255.248
/30	2	255.255.255.252
/31	1	255.255.255.254
/32	0	255.255.255.255