A customer gets a /48 prefix. How many subnets and how big are they?

A /48 leaves 16 bits for subnetting before the /64 interface part — that's 65,536 (2¹⁶) possible /64 LAN segments.

Do the arithmetic on the prefix:

• Customer site prefix: /48
• Standard LAN segment: /64
• Bits available for the customer to subnet: 64 − 48 = 16 bits → 65,536 subnets
• Each of those subnets is a /64 holding ~1.8 × 10¹⁹ hosts

So even a single home or small business getting a /48 has more subnets than it could ever use. (Some allocation schemes split those 16 bits further, e.g. 8 bits for 256 locations × 8 bits for 256 segments each.)

Tip: If a provider only gives a customer a single /64 (instead of a /48), the customer can't subnet at all — a common real-world complaint about stingy ISPs.