Find all devices/hosts?
 

How can I find all the devices(printers,etc...) and computers along with their OS if any installed that are connected in my intranet (devices/computers may be down here)?

I did this earlier using netstat or nmap not sure what I used and how I did.
Can anyone help me?

Nmap will do this.

But if you knew you used netstat or nmap before, and knew they worked, why didn't you just try the commands, or read about what they did, and try it???

When I did them previously they gave me the OS installed as well. But this time they aren't showing.
Also, the same command is showing different outputs in different kernel versions like some showing the MAC addresses some are not.

Read the MAN pages on nmap. That's an option, and will only show up if you enable it. Also, some options only show up if you're running it as root. And remote systems can also be set up to *NOT* show information to remote queries, for security purposes.