One interesting thing about this idea is that, from a higher level, that's exactly how IPv6 works; the "leading zero" is the IP protocol version field. If that

One interesting thing about this idea is that, from a higher level, that's exactly how IPv6 works; the "leading zero" is the IP protocol version field. If that leading zero, er IPv4, is there, the fields of the IP packet are interpreted using IPv4 semantics. If the version field is IPv6, then the IP packet is interpreted using IPv6 semantics. This is how you configure telephone dial plans. Earlier dialed numbers influence the interpretation of later numbers. You dial a 1, an area code is expected next. You dial 9, you get an outline line. Dial plans are a pretty decent setup and allow scoped dialing, but are limited in their extensibility (you can't have a local number start with 1). In IP, the IP protocol version field influences the interpretation of later fields, logically similar to dial plans.