>IPv4 has no equivalent to RAs, so the workaround is to infer prefix assignment > from address assignment. Technically that's not quite correct: RFC-950 defines a subnet mask request/reply. And RFC 1256 defines router advertisements Subnet mask ICMPs were never popular. I think some popular implementation tended to give a wrong reply. Router advertisements were somewhat popular. But got out of fashion when DHCP became the standard way of obtaining configuration.