> I may be getting over my head here)

It may be so, however my own need for clarification was the same as yours. If ISP xyz operates a /16 ip block, and only forwards source packets from its /16... Then, I guess, my question: how do spoofed packers usually travel past their first hops on their route to the target?

[edit] after-thought: I imagine attackers might easily spoof their source as any of their ISPs /16 or /8. Feels like that's not entire story here though.