IPv4/IPv6 subnet check utility.
Usage: subnet <address> [<subnet> [<subnet> [<subnet> ...]]]
  Subnet definition must be in dotted-decimal (IPv4) or in colon-separated
  hexadecimal (IPv6) notation followed by a slash and decimal subnet mask
  or one of the well-known network aliases stored in /etc/cidr.
  If no subnet given, lists all named subnets the address belongs to.
Exit code:
    0         one of the given subnets contains the address
    1         none of subnets contain the address
    3         parse error
    4         internal/system error
    *         other error
Environment:
  CIDR_FILE   path for network aliases file