Under dir \tomcat\bin you will find server.xml file
Change the port in this section:
<Connector port="8400" protocol="HTTP/1.1"
connectionTimeout="20000"
redirectPort="9400" />
to
<Connector port="80" protocol="HTTP/1.1"
connectionTimeout="20000"
redirectPort="9400" />
The ip/domain name should just work if it points to your desktop/server
I just tested it on my laptop and it worked fine after I turned off my
default webserver that was already listening on port 80.
