How is an ISP supposed to know whether the "sender" on an IP packet is legitimate? > That's not related to IPv6. I didn't claim it was. It is, of course, alre

How is an ISP supposed to know whether the "sender" on an IP packet is legitimate? > That's not related to IPv6. I didn't claim it was. It is, of course, already an issue in IPv4 networks. OP was claiming that, once everyone uses IPv6, VPN solutions like Tailscale or WireGuard would no longer be needed. I disagreed because one of main features these solutions provide is access control, i.e. which device gets to connect to which device (and which application) in your private network. Not only is this much more cumbersome to achieve with IPv6 alone (as you need to set up appropriate firewall rules manually on each device), the security guarantees afforded by such firewall rules also won't be as strong as 1) IP spoofing cannot be prevented, 2) encryption is not the default, 3) the attack surface is much larger. > If your ISP allows spoofing addresses This has nothing to do with one's ISP. The ISP cannot possibly know whether the sender IP address on an IP packet is legitimate or not.