Generate and route ULAs without leaking them as global IPv6 space

Frame this document as a source-led configuration and assurance check: Generate and route ULAs without leaking them as global IPv6 space. Only the official source and traced locations below supply facts. Confirm applicability before acting.

Source fact:

The official RFC 4193 — Unique Local IPv6 Unicast Addresses from RFC Editor / Internet Engineering Task Force supports the following bounded statements:

• A unique local IPv6 prefix uses FC00::/7, a local-assignment bit, a 40-bit Global ID, a 16-bit subnet ID, and a 64-bit interface ID. The research record locates this support at Section 3.1 (Format).
• A locally assigned Global ID must be generated with a pseudo-random algorithm to make independent /48 prefix collisions highly unlikely. The research record locates this support at Section 3.2.1 (Locally Assigned Global IDs).
• Exterior routing should ignore and not advertise FC00::/7 by default, while site borders should reject local-address packets unless explicitly routed for a specific /48 or longer prefix. The research record locates this support at Section 4.1 (Routing).

Do not import neighboring assumptions into the source record. The supported task is a scoped comparison involving address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains and the conditions the source actually describes.

What the source does not establish

This RFC evidence supports only the named network-protocol decision; it does not select vendor settings, topology, capacity, or an acceptable failure mode. Do not read the source as proof of implementation or permission to change production. Its guidance remains conditional on DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring and the environment’s recorded constraints.

Applicability questions

• For source statement 1 at Section 3.1 (Format), which observable configuration, record, or test can confirm applicability here?
• For source statement 2 at Section 3.2.1 (Locally Assigned Global IDs), which observable configuration, record, or test can confirm applicability here?
• For source statement 3 at Section 4.1 (Routing), which observable configuration, record, or test can confirm applicability here?
• Within address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, which versions, roles, and configuration states define the review population?
• Could DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring invalidate the test, hide a failure, or change applicability?
• Who owns the decision, and which observation requires stopping, escalation, or rollback?

DSE recommendation:

DSE recommends using the cited source as the evidence anchor for this decision. Use a two-person review for the source interpretation and the resulting operational decision. Record the source location, examined part of address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, observed and expected states, owner, and reason for deviation.

For an approved change, define prerequisites, a limited test path, success and stop conditions, monitoring, and rollback. Check DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring in design order. Protect credentials, keys, recovery material, personal data, and sensitive topology in evidence.

Verification and evidence

Tie each conclusion back to Section 3.1 (Format); Section 3.2.1 (Locally Assigned Global IDs); Section 4.1 (Routing) and to observable material such as configuration snapshots, route or neighbor state, packet captures, counters, topology records, and controlled failover results. Preserve provenance and stable identifiers without copying secrets into the evidence set.

Record the decision even when no change is made, including uncertainty and the next trigger. Use safe testing conditions for disruptive work, preserve rollback proof, and revisit the conclusion after relevant platform, dependency, vendor, or ownership changes.

Official references

• RFC 4193 — Unique Local IPv6 Unicast Addresses — RFC Editor / Internet Engineering Task Force