How do you subnet an IPv6 network using the Subnet ID?

Subnet in the Subnet ID field (between routing prefix and interface ID). With a /48 prefix and 16-bit Subnet ID you get 2^16 = 65,536 /64 subnets — just increment the hextet in hex.

IPv6 Subnetting:

IPv6 was designed with subnetting in mind - much simpler than IPv4.

Standard IPv6 GUA Structure for Subnetting:

|←─── 48 bits ───→|←─ 16 bits ─→|←───── 64 bits ─────→|
┌─────────────────┬─────────────┬─────────────────────┐
│ Global Routing  │  Subnet ID  │    Interface ID     │
│    Prefix       │             │                     │
└─────────────────┴─────────────┴─────────────────────┘
       /48             /64

Subnetting with a /48 prefix:

• Subnet ID field: 16 bits
• Possible subnets: 2^16 = 65,536 subnets
• Each subnet: /64 with 2^64 hosts

Example: 2001:db8:acad::/48

Key insight: Just increment the 4th hextet (Subnet ID) to create new subnets!

Go deeper:

• IPv6 address — subnetting (Wikipedia) — how the subnet-ID hextet between routing prefix and interface ID yields 2^16 /64 subnets from a /48.