NAT routers - is IP spoofing a risk?

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The firewall on my ADSL router sometimes reports stuff like the following:

        Firewall:IP Spoofing detected,from to

(my computer was on at that point).

But is such an attack even a risk on an ADSL router? That is, if the
firewall had not been running, would an ADSL router actually allow WAN-side
traffic through to the LAN just because it claimed to be from an IP address
used by the LAN? It seems illogical that such a device could be fooled,
since WAN traffic is self-evidently WAN traffic regardless of the IP
address it presents to the router, since it arrives on a different physical

I have also heard people say that you should choose a non-obvious address
range for machines on your LAN to guard against spoofing (or attempts to
connect to specific LAN machines by guessing their IP address), but is
there really a risk here or is NAT routing immune to such subterfuges?

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