What is the zone/scope ID (the % part) in an address like fe80::1%eth0?

The %zone suffix tells the OS which interface a link-local address belongs to, because the same fe80:: prefix exists on every interface and would otherwise be ambiguous.

Link-local addresses (fe80::/10) are not globally unique — every interface on a host can have an fe80:: address, and they can even be identical. So when you target a link-local address, the stack must know which link:

• Linux/macOS: ping6 fe80::1%eth0 — the zone is the interface name
• Windows: ping fe80::1%12 — the zone is the interface index number

The %zone is purely local to your machine; it's never transmitted in the packet. Without it, the OS can't decide whether to send out eth0, wlan0, etc.

Tip: You only ever need the %zone for link-local addresses — global addresses are unique, so they route without it.

Go deeper:

• What's that % sign after an IPv6 address? (howdoesinternetwork.com) — a focused explainer of the zone ID with ipconfig examples.