What is subneg mask for 512 subnet?

A subnet mask for a /512 subnet is not valid because subnet masks are defined in powers of two, and the maximum number of hosts in a subnet is limited by the number of bits in the IP address (32 bits for IPv4). The largest valid subnet mask is /32, which allows for a single IP address. However, if you meant a subnet that allows for 512 hosts, the correct subnet mask would be /23, which provides 512 addresses (510 usable for hosts).