CPU410-5H - unable to configure subnet 255.255.0.0 in both X5 and X8 port

Hi all, 

I am facing a problem in configuring the IP address in X5 and X8 port. The detail as below 

Rack 0

X5- 10.50.251.11 , 255.255.0.0

X8- 10.50.225.11 , 255.255.255.0

Rack 1

X5- 10.50.252.12 , 255.255.0.0

X8- 10.50.225.12 , 255.255.255.0

Compilation no error, however when I download, the CPU indicates “inconsistent configuration/specification of the IP address”. Details of diagnostics is “An IP address which is in the same subnet as the IP address of this interface has already been configured/assigned for another interface. “

I do not know what the relationship between X5 and X8 port is. Is it a switch? 

If I assign X8 port subnet to 255.255.0.0, it prompted an error, said:" The subnet addresses of the two IO controllers 'CPU-01-X5' and 'CPU-01-X8' must not be the same."

Can anyone help me to solve this? 

Thank you~