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CIDR

CIDR Stands for classless inter domain routing. It helps in preventing the wasting of IP address. With CIDR, a single IP address can be used to designate many unique IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP network prefix. Eg:192.168.1.0/24

CIDR Notation

CIDR specifies an IP address range using a combination of an IP address and its associated network mask.

CIDR notation uses the following format - xxx.xxx.xxx.xxx/n


Where n is the number of (leftmost) '1' bits in the mask.

For example:  192.168.13.0/24

For class A ..Default subnet mask is 255.0.0.0 so CIDR notation is /8

For class B..Default subnet mask is 255.255.0.0 so CIDR notation is /16

For class C..Default subnet mask is 255.255.255.0 so CIDR notation is /24

 

CIDR and IP Address Aggregation:

IP Address Aggregation means that several networks can be spanned by a single routing entry.

Our router needs to route traffic for six separate networks through the same gateway
(IP address 192.168.1.1):

ip route 172.100.100.0 255.255.255.0 192.168.1.1

ip route 172.100.100.32 255.255.255.0 192.168.1.1

ip route 172.100.100.64 255.255.255.0 192.168.1.1

ip route 172.100.100.128 255.255.255.0 192.168.1.1

ip route 172.100.100.128 255.255.255.0 192.168.1.1

ip route 172.100.100.160 255.255.255.0 192.168.1.1

 

Without CIDR, our routing table would need to maintain a separate entry for each of the six individual networks.

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