CIDR Calculator
The CIDR Calculator enables CIDR network calculations using IP address,
CIDR mask (subnet mask), mask bits, maximum required IP addresses and maximum required subnets.
Results of the CIDR calculation provide the wildcard mask, for use with ACL (Access Control Lists),
CIDR network address (CIDR route), network address in CIDR notation and the CIDR address range for the resulting CIDR network.
For classful subnets, please use the IP Subnet Calculator. For classful supernets, please use the
IP Supernet Calculator. For simple ACL (Access Control List) wildcard mask calculations,
please use the ACL Wildcard Mask Calculator.
Large routing tables have several adverse effects:
-
Routers require more memory in order to store and manipulate their routing tables which increases operation
costs.
-
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 194.1.1.1):
ip route 66.100.50.0 255.255.255.224 194.1.1.1
ip route 66.100.50.32 255.255.255.224 194.1.1.1
ip route 66.100.50.64 255.255.255.224 194.1.1.1
ip route 66.100.50.96 255.255.255.224 194.1.1.1
ip route 66.100.50.128 255.255.255.224 194.1.1.1
ip route 66.100.50.160 255.255.255.224 194.1.1.1
ip route 66.100.50.192 255.255.255.224 194.1.1.1
ip route 66.100.50.224 255.255.255.224 194.1.1.1
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 66.100.50.0 255.255.255.0 194.1.1.1
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:
66.100.50.0 255.255.255.0
would become
66.100.50.0/24
