How do I determine which, if any, static IP addresses are being used on my home WiFi network?

Check the status page of your Router setup.

Well, every network device has a unique identifier called the MAC address (Media Access Control, not an Apple brand ;-). Your Wii, your iPod, your Xbox, network interface cards in your PCs, and your router all have unique MAC addresses that look similar to this: f0:6d:cx:3d:x9:29.

So, as @dpworkin suggested, connect to the admin panel of your router and bring up the list of connected devices. Depending on your router’s firmware, it should show you a listing of connected devices, their hostnames, their IP addresses, and their MAC addresses.

If you see one of your IP addresses is doubled up with two MAC addresses, then your suspicions were correct. Depending on your router, you might then be able to configure it so that it will block any future connections from the rogue device’s MAC address.

If you’re not able to see a single IP address with more than one MAC address associated, then you should audit the MAC addresses of all your personal devices. Any left over from the list will be your rogue device.

We are using DHCP, with the router being 192.168.1.1. The routing table says to start at 192.168.1.2 and ends at 192.168.1.254.
If a device has an address like 10.204.1.10 would it be able to log onto the router?

SRM

That ip series is another internal-use series, just like the 192.168 series. I think that might be your loopback device. Take a look. Do a tracert to localhost and see what the ip addy is.

@srmorgan, the main problem with using a static IP address is if there’s a conflict where two individual devices attempt to use the same IP address. Their traffic gets confused by the machines, packets are “stolen”, and unexpected packets show up on the machine that didn’t request it.

Whether your router will allow devices to connect with a static 10.x.x.x address when the DHCP is serving up 192.168.1.x is probably subject to certain config and security settings. Assuming yours is allowing this, I doubt it’s the primary cause of the problem you’re seeing since your router’s DHCP is only handing out 192.168.1.x addresses. Do you have evidence of 10.204.1.10 (or similar) traffic?

With respect to @dpworkin, I’d honestly be surprised if the loopback (a.k.a. localhost a.k.a. 127.0.0.1) had somehow been changed to 10.204.1.10 (or anything in the 10.x.x.x range). That’d be excessively weird.

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I have noticed on my Windows machines, running a Cisco VPN client for work, that when I get back on my home network (802.11n Apple Time Capsule), that my PC’s network setting (domain, etc.) don’t reset back 100% properly (even after ipconfig /release and /renew). When that problem occurs, I sometimes have to reboot my PC.

Using some advice from a friend I downloaded spiceworks, a network mapping program.
Strange, it gave me addresses that should not be in use on the network. 192.168.1.16, 1,17 and 1.223. No associated MAC addresses and these addresses were not in active use on the network.

I am going to get my children to activate everything they own at one time to see what happens, assuming that they will ever all be at home at the same time. ..

SRM