Classless Interdomain Routing

Classless Interdomain Routing
Classless interdomain routing (CIDR) is an industry standard for displaying the number of
subnet bits used with the IP address of a host or a network. Let’s say you have a 172.16.10.1
address with a 255.255.255.0 mask. Instead of writing the IP address and subnet mask separately,
you can combine them. For example, 172.16.10.1/24 means that the subnet mask has 24
out of 32 bits on.
The following list shows all the possible CIDRs:
0.0.0.0 = /0 255.255.128.0 = /17
128.0.0.0 = /1 255.255.192.0 = /18
192.0.0.0 = /2 255.255.224.0 = /19
224.0.0.0 = /3 255.255.240.0 = /20
240.0.0.0 = /4 255.255.248.0 = /21
248.0.0.0 = /5 255.255.252.0 = /22
252.0.0.0 = /6 255.255.254.0 = /23
254.0.0.0 = /7 255.255.255.0 = /24
255.0.0.0 = /8 255.255.255.128 = /25
255.128.0.0 = /9 255.255.255.192 = /26
255.192.0.0 = /10 255.255.255.224 = /27
Extending IP Addresses 55
Notice that the CIDR list starts at /0 and goes up to /32. With the release of RFC 3021, a /31
can be used on a point-to-point link instead of a /30, which will reduce the number of addresses
used by half. Because a point-to-point connection connects only two systems, there is no reason
to have one address dedicated to the network and one to the broadcast address. Cisco introduced
RFC 3021 support with IOS version 12.0(14)S. You must remember to disable directed broadcasts
with the no ip directed-broadcast global configuration command when using this feature.
A /32 address is designated as a host route and is usually assigned to the loopback interface
on a router.
Let’s now examine how Cisco handles CIDR.