It routes it as an IP4 packet, because it still is a valid IP4 address. Think of it as a way to let a stateless IP4.4 aware NAT router at the edge join two 32-b

It routes it as an IP4 packet, because it still is a valid IP4 address. Think of it as a way to let a stateless IP4.4 aware NAT router at the edge join two 32-bit networks together. An IP4.4 client could send a packet to a destination in that network by putting the address of the IP4.4 router in the usual IP4 destination, and the 'internal' IP address in the IP options header. No 'entire Internet update' necessary