Separate internal routes from border policy
03ULA addresses have global scope but limited routability. Scope is not a promise of Internet reachability, and a default route is not an inter-site agreement.
Separate control-plane example: an exterior ULA prefix advertisement is not learned by default. An advertisement is not the same as the data packet in the previous states.
Source for this drawing: RFC 4193 · Read RFC section 4.1 ↗
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.
Normal IPv6 forwarding operates inside a site; explicit policies control its border.
- Within a site, ordinary IPv6 routing protocols can carry ULA routes without protocol changes. The example assumes installed internal routes.
- Exterior routing sessions default to ignoring received fc00::/7 prefixes and not advertising them. The Border Gateway Protocol (BGP), which exchanges routes between networks, has a border exception using explicitly configured /48 or longer routes.
- Section4.3 recommends rejecting packets with local source or destination at the border unless specific /48-or-longer routing is configured, with appropriate ICMPv6 Destination Unreachable feedback.
Route advertisements and data packets are different objects. The two default stops and the explicit peer path are separate examples. Packet-filter guidance uses lower-case should in the source, not an invented RFC2119 MUST.
Read the RFC section ↗