Sometimes when you issue a ping to a certain host and you do not get a reply, you think the host is offline, but it is actually being blocked by a firewalll. 
What you can do is issue a ping and right after that run the arp -a command:

:/# ping <ip adress host> PING ( 56 data bytes  ^C (abort with CTRL - C)  
:/# arp -a | grep <ip adress host>  <hostname> (<ip adress host>) at (incomplete)  

So, if you do not get a MAC address, the machine is really offline