Interface IP Details lost after reboot

Hello, I just installed FreeBSD. And did ifcondig igb0
route add default

The above IP is dummy.
Then pinged and it works.

Rebooted the server, and tried to ping, it says no route to host.

And ifconfig -a shows igb0 has no IP.

How do I tell FreeBSD to save my IP config forever?
Persistent settings are made in config files, like /etc/rc.conf for the IP address. The Handbook has lots of examples of configuration and settings.
it doesn't let me edit the file. Only shows, but doesn't let me delete or add.

I'm logged in as root only.
Can I edit the file ?
I need to edit that file, as it shows wrong ether name.

My mother board changed, and this new mother board has different ether name.

Can someone please help how do I edit that file ?
Without some details, it is very difficult to tell what is going on. Which editor are you using? What does it do when you try to delete or add? Is there a message or some other indicator?