Since a network interface card can
have both an IPv4 and an IPv6 address as the primary IPs, the standby
network interface card could potentially act as a standby for both
types of primary interfaces.
However, if the IPv4 and IPv6 address(es) are configured
on two separate network interfaces, then the standby interface can
take over the IP address from only one network interface during
a local failover.
That is, IPv4 and IPv6 addresses from two separate network interfaces
are mutually exclusive in a failover condition.