It works because it prevents the ipv6 kernel module from loading, hence disabling any support for IPv6.
Personally, I think a better way, if that's what you want to do, is put "install ipv6 /bin/true" in /etc/modprobe.conf. Don't screw with the
aliases.
What not loading ipv6 does is keep your machine from requesting IPv6 DNS records (AAAA) from your DNS server. Some DNS servers ignore requests for IPv6 addresses, which means the client has to time out the request before it asks for an IPv4 address (A). (If it retries, then it has to
time out all the retries, which could be half a minute or so.) Most DNS servers are well-behaved enough now that they'll return an invalid request result or an unknown result if they don't support IPv6 requests, so the client can make an IPv4 request right away. In other words, for
most people this tip isn't going to improve anything.
FWIW, Google did some initial testing a year or so ago that found a different, but related, problem. If the client is IPv6-enabled, but the Internet connection is IPv4-only, requesting *and*getting* an IPv6 address for a site makes the site completely inaccessible. That's why
they didn't assign an IPv6 address to www.google.com, but instead created a different name (ipv6.google.com) that probably points to the same pool of servers.
pascal Hambourg
Why not just enable IPv6 on Google websites?
We continuously conduct detailed measurements on the quality of IPv6
connectivity, and our latest results show that making Google services
generally available over IPv6 at this time would lead to connection
problems and increased latency for a small number of users. User
experience is very important to us, and we do not want to impact users
on networks that do not yet fully support IPv6. We will continue to
re-evaluate the situation as the IPv6 Internet evolves.
======================================================================
There are lots of IPv6-enabled hosts without global IPV6 connectivity out there. This includes many hosts running GNU/Linux or Windows Vista. If what you wrote were right, *all* these hosts could not reach dual-stack sites, which is fortuntely not the case.
If the client is IPv6-enabled but the host it runs on has no global IPv6 connectivity (i.e. no default IPv6 route or no global address), then non-local IPv6 communications are rejected with a "network unreachable" error, and the client tries again using IPv4.
Only hosts with a broken IPv6 setup, e.g. a default IPv6 route but no good IPv6 connectivity, will experience trouble. This is the "small number of users" Googles talks about.
Things are changing. See
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