Combine aligned blocks
02Aggregation advertises a less-specific prefix that covers several smaller blocks. An exact two-block combination requires equal-sized sibling blocks: adjacent ranges with the same parent bits.
The smallest single prefix containing that pair is /24. It also includes .0–.63 and .192–.255. Those extra ranges are explicit holes in this example, not newly authorized address space.
Source for this drawing: RFC 4632 · Read RFC section 4.1 ↗
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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.
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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.
An aggregate follows binary boundaries; a covering prefix may also include addresses outside the components.
- Hierarchical assignment aligned with the provider topology allows many routes to be represented by a shorter common prefix (§§4.1–4.2). Allocation means giving space for further delegation; assignment means giving space for direct use. Address ownership and network connectivity are different facts.
- Two /26 siblings 192.0.2.0/26 and 192.0.2.64/26 have the same first 25 bits. Their exact union is 192.0.2.0/25. Merely having neighboring decimal ranges does not guarantee an exact single-prefix union.
- Two adjacent /26 blocks .64–.127 and .128–.191 cross a /25 boundary. A single /24 covers them, but also .0–.63 and .192–.255. The drawing marks those extra ranges rather than silently assigning them to the operator.
- An operator needs authority and appropriate routing policy to advertise an aggregate (§5.4). Arithmetic alone is not permission to announce addresses. Proxy aggregation on behalf of others can change traffic paths and needs care.
- §5.4 describes dynamic aggregates generally appearing when at least one component is reachable and disappearing when all are unreachable. An advertised aggregate is not a guarantee that every component is reachable. Manual policy can also govern aggregation.
Blocks and addresses are chosen examples. Bar widths and gaps are proportional to the same /24; no real route announcements.
Read the RFC section ↗