Unique Local Address (ULA) fd00::1
Expand the unique local IPv6 address fd00::1. Understand ULA prefixes (fc00::/7), their similarity to IPv4 private ranges, and when to use them.
Private Addresses
Detailed Explanation
Unique Local Addresses (ULA)
The address fd00::1 expands to:
fd00:0000:0000:0000:0000:0000:0000:0001
ULA Structure
Unique Local Addresses use the prefix fc00::/7, which covers two blocks:
| Prefix | Allocation | Status |
|---|---|---|
| fc00::/8 | Centrally assigned | Not yet defined |
| fd00::/8 | Locally assigned | In common use |
The recommended structure for a ULA:
fd XX:XXXX:XXXX : subnet : interface ID
| |___________| |_____| |__________|
| 40-bit random 16-bit 64-bit
| Global ID Subnet ID Interface ID
Comparison to IPv4 Private Ranges
| IPv4 Range | IPv6 Equivalent | Purpose |
|---|---|---|
| 10.0.0.0/8 | fd00::/8 | Large private |
| 172.16.0.0/12 | (same) | Medium private |
| 192.168.0.0/16 | (same) | Small private |
Key Properties
- Not routable on the public Internet
- Globally unique (when using random 40-bit Global ID)
- No NAT needed within the organization
- Stable across ISP changes (unlike Global Unicast addresses)
- Registrable in local DNS for internal services
Use Case
Use ULA for internal network services that should not be accessible from the Internet: internal DNS, file servers, databases, IoT devices, and inter-site VPN connections. ULA provides stable addressing that does not change when the ISP-assigned global prefix changes.