In Contiki OS, Can a node have more than one IPv6 link-local address? Can a node have more than one IPv6 Unique Link Address (ULA)? Can a node have more than one public IPv6 address?
Yes, yes and yes.
Having multiple addresses on one interface is quite common with IPv6. When using SLAAC your device will probably configure multiple addresses per prefix. Multiple prefixes may be advertised in the Router Advertisement, for example both a normal prefix and a ULA prefix. You can of course also configure addresses manually.
Operating system commonly configure one address per prefix that is stable and doesn't change over time, and then regularly add some temporary addresses that will expire after a while. Those temporary addresses are usually used for outgoing connections so the server you are connecting to can't trace you based on your address.
Although usually an interface only has one link-local address, you can have multiple if you want.
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