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Cisco CLI Error: % Bad mask /xx for address X.X.X.X

When does this error message appear?

This error message appears while attempting to configure an IP Address on a Layer 3 interface on a router or switch running Cisco IOS.

What does this error mean?

The IP Address specified is not a valid host address for the Subnet Mask being used.

What is the most common occurence of this error?

This error occurs most commonly if we attempt to assign a Network Address (or Broadcast Address) to a Layer 3 interface on a router or Layer 3 switch.

Example:

The following example illustrates a common occurence of this error:
172.20.100.96/30
Router1 Router2
Router1 Console
Router1(config)#interface gi0/1
Router1(config-if)#ip address 172.20.100.96 255.255.255.252
% Bad mask /30 for address 172.20.100.96
Router1(config-if)#
The diagram above shows 2 routers connected directly together via a cable. The network segment shared between the 2 routers is 172.20.100.96 /30.
Given a network diagram as shown above, sometimes a student may try to configure Router1's (or Router2's) interface with the IP Address 172.20.100.96. When used with the Subnet Mask 255.255.255.252 (/30), the address 172.20.100.96 is a network address.
A Cisco router will only allow a host address to be configured on a Layer 3 interface. (The only exception is a /31 Subnet Mask as covered in this course.)
What would the correct IP Addresses be for Router1 and Router2's interfaces?Show Answer