There seems to be a lot of wasted breath in these comments about IPv6. Given that its unlikely that your provider even supports IPv6 on their networks, and that globally the traffic via IPv6 is nearly non-existent, I wouldn't spend too too much time caring about it.
Their breath is not wasted! This is an article about securing a Linux server, and explicitly talks about adding a firewall with iptables. If it will only defend against IPv4-enabled attackers, then the server operator is at risk of leaving the system open to compromise.
Learning about ip6tables or turning off ipv6 entirely if not needed would be a better discussion topic, but people here are right to care about a major oversight in the security notes presented.
Less than 2% of traffic to Google is IPv6: http://www.google.com/ipv6/statistics.html Even the attacks CloudFlare sees are mostly DDOS: http://blog.cloudflare.com/ipv6-day-usage-attacks-rise Traffic through Akamai is minimal: http://www.akamai.com/ipv6
Maybe someday it makes sense to spend a lot of time around IPv6 defenses, but today is not that day.