If you need to find a username but only have an ip address, if you use active directory (AD) then you can use the following method to find out the username:
At the command prompt enter the following command:
nbtstat –a ipaddressWhere ipaddress is the ip address.
This will list the machine name using that ip address.
Then run the following command:
net view /domain:ad > somefile.txtWhere ad is the name of the domain you want to search and somefile.txt is the name of the file to contain the output.
This will generate a list of every machine and who is logged in.
Open the output file and search for the machine name determined in step 1 (the username will be listed next to this).