What ISP know about my local network when I use their ONU?

Question

I have FTTP (Fibre to the Premises) and a GPON ONU given to me by my ISP. I know they can see hostnames and MAC of devices connected in ONU. If I connect my personal router to this ONU and then connect a PC to my personal router. Will they be able to see the Hostname and MAC of PC connected to my personal router ? When I go to ONU Admin Webpanel I see only the router hostname and MAC and don't see the devices that are connect to router, do they see the same ?

Answer 1

No, this information is not forwarded through routers.

• MAC address are visible to devices within the same Ethernet network. A router, however, does not directly join two networks – it forwards packets between networks by "regenerating" them with its own MAC address.

• Hostnames are usually collected by a router from DHCP requests. When your devices request an IP address via DHCP, that request is processed by your own router and not forwarded anywhere upstream.

• Hostnames may be collected from mDNS or NetBIOS advertisements. These advertisements use either local multicast or local broadcast, neither of which is normally forwarded by routers.

So the only way your ISP could obtain this information about devices behind your router is by manually accessing the settings panel of your router and logging in with your admin password.

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The GPON ONU/ONT is nothing more than a "fiber modem" (media converter, and occassionally a combo modem/router) – same as an ADSL modem or a cable modem. The device doesn't change the way other routers work, it does not have any way to bypass your router if the router does not allow that otherwise, and it does not have any way to receive information that isn't being sent through your router already.