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Extending IP Addresses

255.255.255.240 = /28

255.255.255.248 = /29

255.255.255.252 = /30

Notice that the CIDR list starts at a minimum of /8 and can't go higher

than /30. This is because the mask must at least be a Class A default, and you
must leave two hosts at a minimum.

Let's now take a look at how Cisco handles CIDR.

Cisco and CIDR

Cisco has not always followed the CIDR standard. Take a look at the way
a Cisco 2500 series router asks you to put the subnet mask in the configura-
tion when using the Setup mode:

Configuring interface Ethernet0:

Is this interface in use? [yes]: return

Configure IP on this interface? [yes]: return

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 subnet-

ting, which does not include the default mask. When dealing with these ques-
tions, 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, /25 is 25 bits.

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 is 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

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 slash notation format. Much better.

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