This is FS#579 carried over from #26:

resolve_prefer_ipv6 = ON
Irssi: Looking up irc.oftc.net
Irssi: Connecting to irc.oftc.net [2605:bc80:3010:101:0:2:0:6667] port 6697
Irssi: Unable to connect server irc.oftc.net port 6697 [Network is unreachable]
resolve_prefer_ipv6 = OFF
Irssi: Looking up irc.oftc.net
Irssi: Connecting to irc.oftc.net [2600:3c03::f03c:91ff:fe96:35cd] port 6697
Irssi: Unable to connect server irc.oftc.net port 6697 [Network is unreachable]

The resolve_prefer_ipv6 setting — also subject of #190 — is not really relevant here. The problem is that for chat servers that advertise AAAA records in DNS, unless Irssi can establish an IPv6 connection to the server, the connection fails. The only way to get it to work is to use /connect -4.

The correct behaviour with getaddrinfo() (which respects the lookup order defined in /etc/gai.conf) would be for Irssi to fall back to using the IPv4 address, if such is advertised (which it is in this case).