Dell PowerEdge C6145 - Manual (Page 89)

Using the System Setup Program | 89
Subnet Mask Configuration
Select Subnet Mask item in the IPMI Configuration screen to view the
following submenu:
Option Description
Subnet Mask Enter Subnet Mask in decimal in the form of
XXX.XXX.XXX.XXX (XXX less than 256 and in
decimal only).
NOTE: When BMC IP status is static, this item is userful.