CTL_NET_IPv4_ADDR_t ctl_net_get_subnet_mask(void);
ctl_net_get_subnet_mask returns the system's subnet mask as set in the CTL_IP_CONFIG_t configuration passed to ctl_net_init or retrieved from a DHCP server.
If IP addresses are assigned by DHCP, ctl_net_get_subnet_mask will returns an all-zero IP address whilst IP negotiation is in progress.
ctl_net_get_subnet_mask is thread-safe.