howto let ivp6 try a conection before ivp4

Both ivp4 and ivp6 activated on wireless ide/card

how to try a ivp6 connection for instance mirrors.mcs.anl.gov
for they are supporting ivp 6

so in general first try ivp6 then ivp4

AFAIK ipv6 will be tried first, when enabled, but I cannot find out where I read that.