Check if mac address is registered I have a device that failed to its DHCP query. If I change its MAC address it works. I suspect a MAC address is already registered somewhere on the network. I know that during a DHCP request the DHCP server test if somebody answer to the registered MAC before the re-affectation. How can we simply check if a MAC address is already registered on the network? Of course I don't have access to the DHCP server neither the network switches. I tried `arp -a` which gives me a big list of IP, but I doubt it's all the IP addresses of my network.

Based on your comment it sounds like some other device on your network has the MAC address that you would like to use for the device you have access to. Finding that other device in the physical world will probably be very difficult to impossible without access to any switches.

NB: If you did have access to the switches, you could follow the MAC tables to a switch port number and from there do a physical cable trace.

To confirm that there is actually another device with the same mac address, you could try using a network auditing tool and attempt to discover all devices on the same broadcast domain.

There is another set of possibilities that involve either static MAC entries either on the DHCP server(s) or some device(s) on the network, or some kind of port security or MAC filtering. It's a little odd to be trying to solve a problem of this nature without access to the switches or servers or access to someone who has access, although not at all unheard of on secure networks.