CIDR - Classless Inter Domain Routing
CIDR - Classless Inter Domain Routing - was adopted to help ease the load imposed on internet and large network backbone
routers by the increasing size of routing tables.
Large routing tables have several adverse effects:
Routers require more memory in order to store and manipulate their routing tables which increases operation
Routing latency is increased due to the large amount of data contained in the routing tables.
Network bandwidth usage is increased by routing updates when routers exchange their routing tables.
A solution to these problems was found in CIDR. CIDR permits IP Address aggregation which in turn reduces the size of
routing tables and so addresses the problems listed above.
CIDR and IP Address Aggregation
So what is IP Address Aggregation? Quite simply, IP Address Aggregation means that several networks can be spanned by a single
routing entry. Consider the following case:
Our router needs to route traffic for eight seperate networks through the same gateway (ip address 184.108.40.206):
ip route 220.127.116.11 255.255.255.224 18.104.22.168
ip route 22.214.171.124 255.255.255.224 126.96.36.199
ip route 188.8.131.52 255.255.255.224 184.108.40.206
ip route 220.127.116.11 255.255.255.224 18.104.22.168
ip route 22.214.171.124 255.255.255.224 126.96.36.199
ip route 188.8.131.52 255.255.255.224 184.108.40.206
ip route 220.127.116.11 255.255.255.224 18.104.22.168
ip route 22.214.171.124 255.255.255.224 126.96.36.199
Without CIDR, our routing table would need to maintain a seperate entry for each of the eight individual networks.
As the eight example networks are contiguous, i.e. their address spaces follow numerically with no gaps, we can
encapsulate all eight with a single CIDR route by simply changing the subnet mask:
ip route 188.8.131.52 255.255.255.0 184.108.40.206
It's easy to see the benefit of IP Address Aggregation and CIDR when we see the difference in routing table entries between
the "before CIDR" and "after CIDR" cases above. This is a very simple example but it is easy to imagine how CIDR can help
in the real world with much larger aggregations.
CIDR brings with it its own simplified form of IP network address notation. Instead of using the network address and subnet mask,
CIDR notation uses the network address followed by a slash ("/") and the number of mask bits. For example, taking the CIDR network
from the above case: