I have a security device that it doesn't respond to icmp and arp request. but i know its MAC address. I want to scan the device with nmap but always says "host down" and does not start port scan (even with -Pn). Also i defined i new arp record for the device but it didn't helped.
Nmap by default performs ARP scan at the discovery stage but if you want to disable it explicitly then you have to specify
--disable-arp-ping. As you said that You have specified
-Pn then it's for sure that port scanning will be done and Host is up should be reported.
Please also tell your Nmap's version so that we can help you even more.