Hi Glen,
The configuration you are doing is little complicated and confusing.
Localhost given as hostname definitely sounds good. In computer networking, localhost is a hostname that means this computer.
Basically the computer is running on an IP address. Best way to check is run in cmd : netstat -aon | find "6400"
This will verify the cms is running on an IP which is assigned to the computer but should not be termed as localhost.
About the Configuration property : These properties are used to Bind the IP/hostname to the servers, give Specific port assigned to the servers.By default installation IP are allotted randomly for all servers except CMS , WACS which is 6400, 6405 respectively. Customer can change this values if they do not want random ports to be used. This concept mainly works when there are more than 1 nodes in cluster and firewall between them so to open all ports between them they assign specific ports to communicate each other.
Now, when you goes into SIA properties and change the CMS port to 6405 this is not correct because default installation give WACS 6405 and when you change it will ask you to assign a value from 1-65535 which is range of ports a computer can have.(Combination of Reserved / Random etc) Check : https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers
Just for reference default ports for BI : CMS (6400) , WACS (6405), SIA (6410) rest are assigned random until changed by the user. Check BI Admin guide for best help.
Hope this will help.
Regards,
Mahak