Global unicast addresses have three parts: the global routing prefix (assigned by your ISP), the subnet ID (you control this), and the interface ID ( bits). A typical allocation gives you a or . With a , you have bits for subnets, allowing subnets each with addresses. You'll never run out within your allocation.
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$ curl repovive.com/roadmaps/network-design/ip-addressing-and-subnetting/ipv6-global-unicast
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