For historical (some would say hysterical) reasons, I am still happily using my Openreach modem for FTTC Fibre Extra with my own (Synology) router in (mesh) Wi-Fi router mode (also several ethernet connections via internal switches).
Writing-up some notes for posterity to document my setup, I see that the ethernet WAN side of the router has been allocated a private network 10 address and the PPPoE tunnel virtual interface has a private 172.16 endpoint address.
Meanwhile, my public IP is in the 87.x.x.x range - as revealed by What Is My IP.
From my internal LAN, traceroute gets to the public IP in one hop and shows a .dyn.plus.net hostname.
Traceroute gets to an Internet IP via the router at 192.168.1.1, the PPPoE tunnel endpoint, and then onwards from
On the router, the routing table shows the PPPoE-WAN interface as the default gateway, as expected.
Q1 - Where is the tunnel actually to? the DSLAM, the local exchange, a Plusnet router?
Q2 - Is there a one-to-one mapping between the PPPoE endpoint address and the public IP, even though it is in a private address range, or else is there some kind of NAT going on out there?
Intrigued as I know double-NAT can cause problems.
Realise at my advanced age that you can think you know a lot but there is always something new to learn - keeps me going since I started with mini-UNIX and PDP11s in the 1970s!
i have now realised that I was missing something that now seems obvious - the source IP address in the IP packets encapsulated by PPPoE - which of course is indeed the public IP address I see from what is my IP.
So the tunnel goes all the way to the ISP and packets already have the correct source IP when extracted. No NAT needed.