Ah, okay. In that case v4 doesn't have a firewall by default either. That's precisely why routers come configured with a firewall that blocks inbound connectio

Ah, okay. In that case v4 doesn't have a firewall by default either. That's precisely why routers come configured with a firewall that blocks inbound connections from the WAN -- because the protocol itself doesn't have a firewall by default, and neither does NAT.