I have FTTP and a smarthub 2 only connected to the ONT.
My Synology disk station says I have double NAT.
On investigation the router homepage states the default gateway is 172.16.x.x sending me a WAN IP address. The SmartHub is providing LAN IP addresses in 192.168.1.x range.
I assume this is normal and there is nothing I can do about this? Is it a consequence of FTTP pppoe connections?
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The 172.16 .xxx.xxx address is the address of the BT BRAS (Broadband Remote Access Server), it will be the same regardless of whether you are on FTTP or FTTC. Because it is a private address rather than a public routeable address your device thinks you are double natting.
If the Synology is getting its IP address using DHCP, then its default gateway should be 192.168.1.254, which would be from the home hub.
You could of course set the Synology to manual setting, with an IP address outside of the DHCP range, like 192.168.1.50. Then you can set both the default gateway and DNS server addresses to 192.168.1.254.
As @licquorice has said, the home hub would be seeing its gateway address from the next router in the network.
Its just surprises me that the Synology can see the BRAS IP address, as what it should be seeing is your public IP address which would be the case if port forwarding is needed.
If it cannot see your public IP address, then you are not going to be able to access it remotely, if that is your intention.
There is no problem accessing the DS remotely via Synology connect, it just sees 2 routers in the network for some reason..