(editing my previous response)
It looks like this is part of the web.conf in full now:
# Set to "yes" to enable IPv6 support, or "only" accept connection
# on IPv6 only
listenOnIPv6 = yes
Putting this here since this thread is the top result for the search. 😕
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I figured I'd update this, since this question has a lot of views...
Splunk does support IPv6 for communication, however it's not switched on by default. You probably cannot switch IPv4 off completely, but there's no need to give your server a v4 address just because v4 is enabled in Splunk.
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