Filter private IPv4 prefixes at public routing boundaries

Use this document to resolve one bounded operational decision: Filter private IPv4 prefixes at public routing boundaries. Only the official source and traced locations below supply facts. Confirm applicability before acting.

Source fact:

The official RFC 1918 — Address Allocation for Private Internets from RFC Editor / Internet Engineering Task Force supports the following bounded statements:

• The private IPv4 blocks are 10/8, 172.16/12, and 192.168/16; their addresses are unique only within cooperating private networks. The research record locates this support at Section 3 (Private Address Space).
• Private routing information must not cross inter-enterprise links, and packets with private sources or destinations should not be forwarded across those links. The research record locates this support at Section 3 (Private Address Space), routing boundary rules.
• Enterprise border routers should filter packet and route leakage in both directions, including inbound private routes that would point private space outside the enterprise. The research record locates this support at Section 5 (Operational Considerations).

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 (Private Address Space), which observable configuration, record, or test can confirm applicability here?
• For source statement 2 at Section 3 (Private Address Space), routing boundary rules, which observable configuration, record, or test can confirm applicability here?
• For source statement 3 at Section 5 (Operational Considerations), 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.

An implementation decision needs an owner, approved window, prechecks, observable outcome, stop authority, and rollback path. Validate DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring before and after the test, and store only sanitized operational evidence.

Verification and evidence

Tie each conclusion back to Section 3 (Private Address Space); Section 3 (Private Address Space), routing boundary rules; Section 5 (Operational Considerations) 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.

Preserve both successful and failed observations, along with approval and rollback evidence. Avoid uncontrolled production experiments. Assign an expiry or event-driven recheck so the conclusion is not treated as permanent.

Official references

• RFC 1918 — Address Allocation for Private Internets — RFC Editor / Internet Engineering Task Force