Recurly does not have support for IPV6 today, as many payment gateways do not support it either. We recommend truncating the IP value to the max allowed, which will prevent any transaction declines. For now, it'll be best to truncate at 20 characters. IPV6 is getting more common but not yet standard.
We are closely monitoring the need for IPv6 as more payment gateways support it in their code base.