If someone plugs in a modem into my modem, can i see their ips on my Browser?

If a ROUTER is connected to YOUR router, you will see only ONE IP for the connected router. You won't see any of the IP addresses of the devices connected to the added router. Don't confuse routers with modems, they are radically different devices and serve different functions. I do know SOME modems are built into routers, those are normally for DSL or Cable and the router functions are actually separated from the modem functions. (They are that different.) You can see how a router works at https://computer.howstuffworks.com/router.htm

yes.

A modem connects between the ISP line and the rest of your equipment. It converts between analogue and digital signals. You cannot plug one modem into another modem.

You are probably talking about routers rather than modems.

If you connect A PC/LAN port on your ISP router to the WAN port on another router. If you are connected to the ISP router all you can see is the IP address the second router is using on the ISP router.

If you are connected to the second router, then you can access the IP tables of both routers if you have the appropriate passwords. You can also try pinging all the addresses on the routers to see what devices respond. No response does not mean that a device is not using the address. It could mean that the device is configured to ignore pings.

I hope this helps.

Hi no this does not tell you anything. the modem should have a label that gives some indication of who supplies it.