RFC 3986 allows three kinds of host - a registered name, an IPv4 address, or an IPv6 literal - and it helps to know which one you have. Today you write a classifier that labels the host, pinning the boundary where a dotted-decimal string stops being a valid IPv4 address.
Classify a host as a registered name, an IPv4 address, or an IPv6 literal.
RFC 3986 says a host is one of three things: an IP-literal (the bracketed IPv6 form from last lesson), an IPv4 address in dotted-decimal, or a registered name - and knowing which lets a program decide whether to resolve a name or connect to an address directly. The bracketed case is unmistakable. IPv4 is a stricter pattern: exactly four parts separated by dots, each a number from 0 to 255. Everything else is a registered name, which is the permissive catch-all.
The instructive edge is 256.0.0.1. It looks like an IPv4 address, but 256 is out of range for an octet, so it fails the IPv4 grammar. It does not become an error, though - a registered name is allowed to contain digits and dots, so 256.0.0.1 is simply a (perhaps unresolvable) registered name. That fall-through is the whole point of the classification order: try IPv6, then IPv4, and let anything that matches neither be a reg-name. Check the octet upper bound at exactly 255, so 255 passes and 256 falls through - the boundary is where the classification flips.
type HostKind intconst ( RegName HostKind = iota; IPv4; IPv6 )func classifyHost(h string) HostKind {if strings.HasPrefix(h, "[") { return IPv6 }// IPv4 = four dot-separated octets, each 0..255// anything that fails that is still a legal reg-name}