This is the subnet mask paired with the IP address in this row, expressed as an address with the network bits set to 1 and host bits set to 0. Admins use it together with the entry's address to determine which subnet the array's interface belongs to, which affects how routing and access rules treat its traffic. A mask of 255.255.255.0 indicates the interface sits on a /24 network alongside other hosts sharing the first three octets.