Keep local addresses when sites or providers change
04Independent local prefixes can survive an Internet Service Provider (ISP) change. When sites join, distinct prefixes help avoid renumbering, but routes still need configuration.
Separate renumbering event: the provider-based prefix changes, while Site A’s local prefix and the configured local path stay the same. This is not a promise to preserve provider-address sessions.
Source for this drawing: RFC 4193 · Read RFC section 4.2 ↗
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.
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“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
High probability of uniqueness helps interconnection; it is not a mathematical guarantee.
- Each site can generate a separate /48 without a central assignment service. Section3.2.3 explicitly acknowledges possible collisions.
- For distinct local prefixes, merging sites usually avoids ULA renumbering. Routes to each prefix still have to be configured; merely drawing a tunnel does not create them.
- Changing provider-based global addresses does not itself change the local prefix. Existing communication using local addresses can remain independent of that provider renumbering.
These are two deliberately distinct illustrative prefixes, not a collision simulation. VPN means Virtual Private Network; secure tunnel establishment is outside this address-format RFC.
Read the RFC section ↗