The CIDR Calculator enables CIDR network calculations using IP address, 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), , network address in CIDR notation and the CIDR address range for the resulting CIDR network.
For classful subnets, use the IP Subnet Calculator. For a simple smart-phone optimized CIDR Calculator, visit CIDR Calc. For ACL (Access Control List) wildcard mask calculations, use the ACL Wildcard Mask Calculator.
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:
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.
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 separate networks through the same gateway (ip address 19.1.1.1):
ip route 66.10.5.0 255.255.255.224 19.1.1.1 ip route 66.10.5.32 255.255.255.224 19.1.1.1 ip route 66.10.5.64 255.255.255.224 19.1.1.1 ip route 66.10.5.96 255.255.255.224 19.1.1.1 ip route 66.10.5.128 255.255.255.224 19.1.1.1 ip route 66.10.5.160 255.255.255.224 19.1.1.1 ip route 66.10.5.192 255.255.255.224 19.1.1.1 ip route 66.10.5.224 255.255.255.224 19.1.1.1
Without CIDR, our routing table would need to maintain a separate 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 66.10.5.0 255.255.255.0 19.1.1.1
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:
66.10.5.0 255.255.255.0would become
66.10.5.0/24
We have now made a smartphone optimized version of this calculator available at CIDR Calculator