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What would happen if I assign my router a random static WAN IP from my ISP’s subnet range, assuming that this particular IP is not currently used by any subscriber? Would it work, given there is no conflict with another subscriber? My understanding is that this IP could just have been allocated to me by ISP’s DHCP server,so it should work.

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Your ISP allocates you a range of ipv6 subnets via DHCP-PD (e.g. a "/56", a "/60", etc).

If you send an IPv6 packet with a source address outside of this "/56" via your ISP gateway, it may just ignore it, or most likely reject it: you will receive in a ICMPv6 packet indicating a routing problem.

Same for IPv4, with a ICMP packet indication a routing problem.

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In general, I wouldn't expect this to work, but mainly because of deliberate security features, not because of technical reasons. (Although this depends on ISP, and probably on the access technology as well.)

I'd expect that most security-conscious ISPs configure layer-2 isolation of some sort, and only send packets through your link after verifying that the address matches what the DHCP server has assigned to you (port isolation, DHCP snooping, or various other names). But it's possible that an ISP would forget to do this, or not even have the technical ability.

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