you can either control this yourself or let the OS decide. Refer to the splunk-launch.conf:
Specifies an interface that splunkd and splunkweb should bind to, as
opposed to binding to the default for the local operating system.
If unset, Splunk makes no specific request to the operating system when
binding to ports/opening a listening socket. This means it effectively
binds to '*'; i.e. an unspecified bind. The exact result of this is
controlled by operating system behavior and configuration.