Read the local IPv6 address fields
01A Unique Local Address (ULA) is a 128-bit IPv6 unicast address: one destination interface. Its prefix is intended for local communication, not ordinary public Internet routing.
Illustrative Subnet ID 0001 and Interface ID 0000000000000001 produce fd12:3456:789a:1::1. No interface identifier assignment procedure is implied.
Source for this drawing: RFC 4193 · Read RFC section 3.1 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
The L bit selects locally assigned fd00::/8 inside fc00::/7.
- The fields are a 7-bit fc00::/7 prefix, 1-bit L, 40-bit Global ID, 16-bit Subnet ID and 64-bit Interface ID.
- L=1 identifies locally assigned prefixes. RFC4193 leaves L=0 for a possible future allocation method; it does not define a local fc00::/8 allocator.
- The first 48 bits identify the locally generated prefix; a chosen 16-bit Subnet ID extends it to /64. The interface completes /128.
All addresses here are fictional, fixed worked examples, not recommended prefixes or a generation service. Field widths in the illustration are schematic.
Read the RFC section ↗