If you have a home network, then every device on that network will have an IP address. I just counted up all the devices on my office LAN and there are 18 of them! A desktop PC, a laptop, a couple of CCTV cameras, the router, a phone, a clever little device for monitoring temperatures around the house, and more besides.
With all that lot, it was definitely time to make a neat list of devices, and tidy up their allocation of IP addresses.
Most devices get their IP address from your router. The router has what's called a DHCP server, which is capable of allocating an IP address to any device that asks for one. And most devices have the ability to contact the default DHCP server, ie your router, to ask for one. Chances are, your router will probably be configured to hand out addresses in the range of something like 192.168.1.1 to 192.168.1.200.
So, how to make a list of all the IP devices on your LAN? If they all use DHCP, then just log into your router and ask it for a list of all the DHCP clients that are currently connected. But if they don't all use DHCP, or you're not sure, then the router will miss some. Which is why it can be useful, fun and educational to run a network scanning tool occasionally.
These tools work by trying to harmlessly connect to each IP address within the range you specify, and then producing a report. And my tool of choice for this is Advanced IP Scanner. You can get it from http://www.radmin.com/products/ipscanner/, it's about 12 MB, and runs on all versions of Windows (32- and 64-bit) from XP onwards.
Just fire it up, give it a range of IP addresses to scan, and away it goes.
An important word of warning, though: Don't point such tools at ranges of public IP addresses (ie those outside your own LAN), and don't try running such a tool on your office network without the knowledge and permission of your admin guys. It'll be regarded as a hacking attempt, and could lead you into all sorts of trouble.