Security → What is the unknown device connected to my speedtouch modem?

My internet connection consists of a Thomson speedtouch st516 modem connected to a Linksys wrt54gl router. When I log into the speedtouch interface via 10.0.0.138 (the ip I assigned it), then click on Home Network/Devices, I see the detected devices as follows:
1. speedtouch 192.168.1.254
2. my computer with properly assigned internal ip address
3. Unknown-00-23-69-16-18-bb 10.0.0.137

What is #3? I don't recognize the mac address and have not assigned that IP, nor is it listed anywhere on my router. How can I find out what it is or get rid of it?

Are you by chance on a laptop and it has the feature of becoming a hot spot sharing your connection?
Could be another wireless adapter.

If you are sure that no one connected a device to a lan port or connected via wireless, it could be that someone logged in via wireless because of a weak password. Turn wireless off or assign a better password.

Comcast hotspot ?

x

00-23-69 is a MAC block assigned to Cisco/Linksys.

If the modem has a 10.x.x.x address, the router you've connected to the modem likely needs a 10.x.x.x address on its WAN port. i.e., chances are good that it's your own Linksys router. You ought to be able to look at the router and find out its WAN MAC address.

But I'm confused about your network - if the Linksys is in routing mode with NAT enabled, how's your computer address showing up at the modem?

OMG, yes, you are right, it is my WAN port. Doh.

As for your other questions. *shrug*, I know very little about networking.

Thanks for your help.

Edited to add: On second thought... the IP indicates it was my computer when connected via ethernet cable... although when I was checking that modem page I was connected via wireless and had a different IP. Very confused how that could be the case...

"Computers" don't have IP addresses. Network interfaces are the things that have IP addresses.

The WiFi interface and the Ethernet interface on your computer are different interfaces...

When I connect my laptop via ethernet the private IP address and/or mac address is different than the wireless connection on the same device. dave explained why they are different.