ipv4 definition-or-command argument...
ipv6 definition-or-command argument...
Without a modifier, interface and router definitions and commands that come before either will be applied to both IPv4 and IPV6. Commands within an interface or router assume the same behaviour as the enclosing definition.
When preceded by a modifier, the command or definition can be made to apply to IPv4 or IPv6 only. Note that you cannot create an IPv4 only command within and IPv6 interface or vice-versa.
interface eth0 myboth src4 192.0.2.0/24 src6 2001:DB8::/24 ipv4 server http accept ipv6 server http accept ipv4 interface eth0 my4only src 192.0.2.0/24 server http accept ipv6 interface eth0 my6only src 2001:DB8::/24 server http accept
Many definitions and commands have explicitly named variants (such as router4, router6, router46) which can be used as shorthand.