"The given IP address or range is invalid." error in Add a New IP Address in WHM

Symptoms

On the Add a New IP Address page in WHM, you try to add an IP address in the New IP or IP range to add: field and get this error:

The given IP address or range is invalid.

Description 

The error appears if you input a single IP address with a netmask instead of a network address, for example, 192.0.2.3/24. The error also appears if you input a network address with a netmask of less than 24 or more than 30 bits, for example, 192.0.2.0/23 or 192.0.2.0/32.

Workaround

Enter just the IP address without the netmask in the New IP or IP range to add: field, then select the corresponding netmask from the Select a subnet mask for the IP or IPs above to use: dropdown menu. For example, to input the 192.0.2.3 IP address with a /24 netmask, enter 192.0.2.3 in the New IP or IP range to add: field and select 255.255.255.0 from the Select a subnet mask for the IP or IPs above to use: menu.