alias addresses both ipv4 and ipv6

Hello,
sorry for the newbie question. We live in the age of migrations.

I need to have both an IPv4 and an IPv6 alias on the same net
interface, and I am unsure of the numbers in the option names.
If I screw this up I'll lock myself out of the system, so I
better get it right.

This is the networking part of my rc.conf now, with the obvious
obfuscations.


ifconfig_vtnet0="DHCP -tso -rxcsum -txcsum"
#ifconfig_vtnet0_alias0="123.123.123.161/17"
ifconfig_vtnet0_ipv6="inet6 accept_rtadv"
ifconfig_vtnet0_alias0="inet6 dead:beef:dead:beef::2 prefixlen 64"

The question is how to add the commented out line.
Should it be alias0 (no conflict with the ipv6?), alias1 or what?

I have read the rc.conf manpage, and sadly it doesn't really answer
this. It discusses each of ipv4 and ipv6 in isolation.

-- 
Ian