Reference Guide: Shared Web Server Panel
In most scenarios, it is not necessary to configure an "Internal Host" in your Atom/Molecule/Cloud.
If left blank, the jetty server associated with your web services, will listen on all network interfaces configured on your machine ( 0.0.0.0 ). There is usually only one NIC configured, so this is usually OK.
In some circumstances, a machine may have multiple network adapters, and can therefore by on several networks. A good "Boomi" example of this, would be if you have configured your Molecule's clustering traffic to go over a private network, while allowing your primary integration traffic to have access to your wider corporate network.
In this "multi-homed" scenario, you may want your server to only listen on that primary network adapter. This is when you should configure that Internal Host value.
Reference Community Idea: Bind molecule's listener to a specific network adapter
In a Molecule configuration, you may need to create a new host alias that resolves to the primary IP address of each node in your cluster. For example, you could modify /etc/hosts on each node, and add the alias to the line containing that primary address. On each machine, that alias would resolve to the IP of THAT machine. You can then use the alias, as the value of the Internal Host in Shared Web Server Settings.