You can also find the used IP Addresses on your network by use of GUI tools. There are many tools that can do that, but I prefer 2 nice, small and most important - free - tools:
Net World Scanner
Small, free and simple - this tool will list all IP addresses on a given subnet (even remote ones). It will also list the NetBIOS name of the computers, and if any shares are present, double-clicking a computer will list the shares on that computer (providing you have the right access permission - this is not a hacking tool).
You can also create a shortcut to the tool and in the Target box add /subnet:w.x.y to quickly scan a specific subnet.
Download it from HERE (156kb).
TCP Net View
Small, free and useful on local networks. It will list computer NetBIOS names, IP addresses, MAC addresses and remarks, plus will export to a test file.
Another cool method to quickly resolve LAN IP addresses is by using this nifty tool by Nir Sofer:
FastResolver is a small utility that resolves multiple host names into IP addresses and vice versa. You can simply type the list of IP addresses or host name that you want to resolve, or alternatively, you can specify IP addresses range that you want to scan. For local network, FastResolver also allows you to get the MAC address of all IP addresses that you scan. FastResolver is a multithreaded application, so it can resolve dozens of addresses within a few seconds.
FastResolver - Host Names/IP Addresses/MAC Address Scanner