Thank you for your reply!
It's so good for the four ways you gave me to detect FreeNAS's IP address.
I want to find a best way for newbie of FreeNAS. Maybe he/she don't know how to use a router and have no monitor. Unfortunately, he/she also have no smart mobilephone, like iphone or android phone. In this case, how could they get FreeNAS's IP?
You seem to be looking for an easy way for a non-tech-savvy person to find out the IP address of the FreeNAS box. But therein lies the problem, those aren't the people that FreeNAS is targeting and are the last people that should be using FreeNAS.
If you aren't technical enough to know how to use a router, you aren't technical enough to be running FreeNAS safely. If you don't have the spare time to plug it into a monitor for long enough to get the IP Address, you don't have the spare time to learn how to properly use FreeNAS safely and the best practices.
And as fracai said, anyone who has done their due diligence should know that the FreeNAS advertises as freenas.local on default installation, and with a properly set up network you should be able to access the FreeNAS by that address and get the IP address from that information as well.
You are looking for a solution to a problem that does not exist. If it does come into existence one day, it is a problem between keyboard and chair.
If none of the options presented in this thread are easy enough for a user, then FreeNAS itself is not going to be easy enough. The learning curve to simply use FreeNAS for a user that can't figure out how to do networking 101 would be nigh insurmountable. Learn how to do
networking before setting up
Network Attached Storage.