What is CIDR
CIDR (Classless Interdomain Routing) that allows internet service provider to reduce the wasting of ip address by assigning a subnet to a company instead of the entire network. It also reduce the size of internet routing tables in the router.CIDR allows routing protocols to summarize multiple networks in a single route.With CIDR ip address and their subnet mask are written as four octet ,separated by period and follwed by a forward slash that represent the number of subnet mask.Example -172.16.1.0/24 is a Class B subnetted address and 24 shows the number of network bit in this address and the subnet mask would be 255.255.255.0.We write the CIDR value according to the number of bits available for the network part. For example-
255.0.0.0-netwoek bit is 8 so CIDR value is /8
255.128.0.0-network bit is 9so CIDR value is /9
255.240.0.0-network bit is 12 so CIDR value is /12
255.255.240.0-network bit is 20 so CIDR value is /20.
255.255.255.252-network bit is 30 so CIDR value is /30
When we see subnet mask of slash notation (CIDR), we can know the following thing-
/28
Means network bits are 28-11111111 11111111 11111111 11110000
Subnet Mask- 255. 255. 255. 240
In the last octet 4 bits on and 4 bits off.
Block size – 2n, where n is the number of bits of host part-24-16
Number of the subnets-2n where n is number of bit borrowed.
Number of the host-2n-2 – 24-2 – 14
/29
Means network bit are 29 – 11111111 11111111 11111111 11111000
Subnet Mask- 255. 255. 255. 248
In the last octet 5 bits on and 3 bits off.
Block size – 2n, where n is the number of bits of host part -23-8
Number of the subnet-2n, where n is number of bit borrowed.
25=32
Number of the host- 2n-2=23-2=6
/30
Means network bit are 30 – 11111111 11111111 11111111 11111100
Subnet Mask- 255. 255. 255. 252
In the last octet 6 bits are on and 2 bits are off.
Block size- 2n, where n is the number of bit of host part – 22-4
Number of the subnet -2n, where n is the number of bit borrowed.
26=64
Number of the host -2n-2=22-2=2
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