Incorrect subnet mask causing routing issues

Symptoms

Servers with several IPs can experience network issues if the subnet mask doesn't match between all IP addresses in the same subnet. This can include routing problems that prevent your server from communicating with other servers, intermittent network resolution, or even broadcasting the wrong primary IP address, leading to the inability of your server to make outgoing connections, receive incoming connections, or even license validation issues with the cPanel software.

Description 

When several IPs from the same subnet are present within the server's network configuration, they should all use the same subnet mask to ensure no issues occur. You can use the following command to verify your server's configured IP addresses and the corresponding subnet masks:

# ip addr | grep inet
inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host
inet X.X.X.215/32 brd X.X.X.X scope global dynamic eth0
inet X.X.X.216/32 brd X.X.X.X scope global dynamic eth0
inet X.X.X.217/24 brd X.X.X.X scope global dynamic eth0
inet X.X.X.218/32 brd X.X.X.X scope global dynamic eth0

Please note that in the above example, the IP X.X.X.217 has a subnet mask of /24, differing from the others of /32. Your hosting provider will be able to provide you with the recommended subnet mask of any given IP address based on how their own networking environment is set up.

Workaround

Ensure that all IPs from the same subnet are configured to use the same subnet mask. Detailed changes to your server's networking configuration should be handled through your System Administrator, with guidance from your server's hosting provider. WHM does not include any tools to modify the configuration of existing IP addresses.