Cisco and CIDR

Cisco and CIDR
Cisco has not always followed the CIDR standard. Take a look at the way a Cisco 2500 series
router with a legacy version of IOS asks you to put the subnet mask in the configuration when
using the Setup mode:
Configuring interface Ethernet0:
Is this interface in use? [yes]: <Enter>
Configure IP on this interface? [yes]: <Enter>
IP address for this interface: 1.1.1.1
Number of bits in subnet field [0]: 8
Class A network is 1.0.0.0, 8 subnet bits; mask is /16
Notice that the router asks for the number of bits used only for subnetting, which does not
include the default mask bits. When dealing with these questions, remember that your answers
involve the number of bits used for creating subnets, not the number of bits in the subnet mask.
The industry standard is that you count all bits used in the subnet mask and then display that
number as a CIDR; for example, the /16 the 2500 reported back to you takes into account all
16 bits that are set to 1.
The newer IOS that runs on Cisco routers, however, runs a Setup script that no longer asks
you to enter the number of bits used only for subnetting. Here’s an example of a new 1700 series
router in Setup mode:
Configure IP on this interface? [no]: y
IP address for this interface: 1.1.1.1
255.224.0.0 = /11 255.255.255.240 = /28
255.240.0.0 = /12 255.255.255.248 = /29
255.248.0.0 = /13 255.255.255.252 = /30
255.252.0.0 = /14 255.255.255.254 = /31
255.254.0.0 = /15 255.255.255.255 = /32
255.255.0.0 = /16

Subnet mask for this interface [255.0.0.0]: 255.255.0.0
Class A network is 1.0.0.0, 16 subnet bits; mask is /16
Notice that the Setup mode asks you to enter the subnet mask address. It then displays the
mask using the CIDR slash notation format. Much better.