The /48 IPv6 Prefix: Site-Level Allocation

The /48 IPv6 Prefix

A /48 prefix is the standard allocation for a single site (office, campus, or home) in IPv6. It provides an enormous amount of address space — 65,536 individual /64 subnets.

Key Details

• Prefix Length: 48 bits
• Remaining Bits: 80 (16 for subnetting + 64 for host IDs)
• Available /64 Subnets: 65,536 (2^16)
• Total Addresses: 2^80 (approximately 1.2 x 10^24)

Address Structure

For the prefix 2001:db8:abcd::/48:

2001:0db8:abcd : SSSS : HHHH:HHHH:HHHH:HHHH
|____ /48 ____| |sub | |_____ host ID _____|

• First 48 bits: Network prefix (assigned by ISP)
• Bits 49-64: Subnet ID (16 bits, managed by the site)
• Bits 65-128: Interface identifier (64 bits, per-host)

Why /48?

The /48 allocation was chosen because:

1. 65,536 subnets is more than enough for any site, from a home to a large campus
2. It leaves the 16-bit subnet field for clean, hierarchical internal addressing
3. It maintains the sacrosanct /64 boundary for the interface identifier

Internal Subnetting

With 16 subnet bits, administrators can create a clean hierarchy:

2001:db8:abcd:0001::/64  -> Building 1, Floor 1
2001:db8:abcd:0102::/64  -> Building 1, Floor 2
2001:db8:abcd:0201::/64  -> Building 2, Floor 1

/48 vs. /56

Some ISPs assign /56 prefixes to residential customers (256 /64 subnets) and reserve /48 for businesses. Both are valid under current IANA guidelines. A /48 provides much more flexibility and is recommended for any non-trivial network.

Requesting a /48

Organizations receive /48 allocations from their ISP or directly from a Regional Internet Registry (RIR) as part of a larger allocation. Most ISPs will assign a /48 to business customers upon request at no additional cost.