Hi I wonder if anyone can help me. I have a device on my network which I have no idea what IP address it has, I have no instruction on how to reset the IP address, no software either.
Can I communicate with this device via the MAC address only?

Realistically not you cannot. You may however be able to determine it's ip address by use of the "arp -a" command from a machine on the same network. The device may well report it's ip address this way.

This is a really odd request. How can a device have an unknown IP address? And it has no software? Whoa...

But I would log on to a box on the same subnet as this mystery box and ping the broadcast address. It should respond. If it is talking to the outside world (beyond the local subnet) it is talking via a router that must know its IP/MAC address pair. Dumping the arp tables on the routers should do it.

Another thought... unplug it from the network. If anyone complains about a box that went down contact them for more info. If this box is not in DNS how do folks access it? It may not be in use at all.

All,
Being a N/W developer, usually I face this problem. There is very simple solution for this.
1> Connect that device with a "Cross-over" N/W cable to another computer.
2> Start a N/W packet capture utility (tcpdump/ethereal/snoop) on that computer
3> Restart device
4> When device start it sends ARP broadcast containing its own IP-Address. This is done to ensure that no other device is assigned same IP.

5> After getting that packet with IP-Address (Example 192.168.2.13) you can assign your computer a smallest possible subnet mask (255.255.255.252) with another IP in that "smallest" Network (Example 192.168.2.14/255.255.255.252)
6> Now access that device with any of login/configuration service

All the Best!!!

Just curious.... what is 'N/W'? And what does a 'N/W Developer' do exactly?

N/W -> NetWork :-)

Ah, good to know - thanks!