Metadata in the traffic header (Server-NAT-host)?

Question

Traffic analysis

Good day.
Question: How are traffic headers marked that are destined for a specific host that sits behind NAT? Is the traffic tagged by the router or the server?
In simple terms, does the ISP see the mac address of the host behind the router?
Thank you in advance!

Answer 1

The traffic is not marked, the router itself stores in memory a table with addresses and ports where and from where the traffic was NATed, this data is enough for the router.
MAC addresses are visible only within the network segment, i.e. MAC addresses from the local network are not visible outside the router, the same is true in the opposite direction - MAC addresses from the provider's side are not visible on the local network.