Here's an example of what I have in /etc/rc.conf on one of my machines. Your interfaces and addresses should be different.
#static network configuration ipv6_enable="YES" network_interfaces="lo0 igb0" ipv6_network_interfaces="lo0 igb0" # ifconfig_igb0="inet 128.182.145.59 netmask 0xffffff00" ifconfig_igb0_alias0="inet6 2001:5e8:1:91:225:90ff:fe05:1dc2 prefixlen 64" defaultrouter="128.182.145.1" ipv6_defaultrouter="2001:5e8:1:91::1" #
