Adding an IPv6 address is similar to the mechanism of "IP ALIAS" addresses in Linux IPv4 addressed interfaces.
Usage:
# /sbin/ip -6 addr add <ipv6address>/<prefixlength> dev <interface>
Example:
# /sbin/ip -6 addr add 2001:0db8:0:f101::1/64 dev eth0
# /sbin/ifconfig <interface> inet6 add <ipv6address>/<prefixlength>
# /sbin/ifconfig eth0 inet6 add 2001:0db8:0:f101::1/64