Assuming you are using zen detection, you can disable the multi-cast option and set the addresses of the unicast nodes.
If you use the default port 9300 on your hosts, the settings below should work in your configuration file
# disable multicast
discovery.zen.ping.multicast.enabled: false
# set the list of hosts for unicast discovery
discovery.zen.ping.unicast.hosts: ["xxx.xxx.xxx.xxx", "yyy.yyy.yyy.yyy"]
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