CIDR Aggregation and Supernetting
How to combine multiple smaller subnets into a single larger CIDR block (supernetting / route summarisation) and why it matters for routing efficiency.
What is supernetting?
Supernetting (or route summarisation / aggregation) is the opposite of subnetting. Instead of splitting a network into smaller pieces, you combine multiple smaller networks into a single larger one by shortening the prefix length.
This is crucial for keeping BGP routing tables manageable. ISPs aggregate customer routes so they only need to advertise one large prefix instead of hundreds of smaller ones.
Example: Aggregating four /24s into a /22
These four consecutive /24 subnets can be summarised as a single /22:
| Individual /24 | Binary (last two octets) |
|---|---|
| 192.168.0.0/24 | 00000000.xxxxxxxx |
| 192.168.1.0/24 | 00000001.xxxxxxxx |
| 192.168.2.0/24 | 00000010.xxxxxxxx |
| 192.168.3.0/24 | 00000011.xxxxxxxx |
The first 22 bits are identical across all four. So they summarise as:
1,022 usable hosts — covers all four /24s
The aggregation rule
Two networks can be aggregated into one if:
- →They are the same size (same prefix length).
- →They are adjacent (no gap between them).
- →Together, they form a contiguous block that starts on a power-of-two boundary.
Examples:
| Networks | Can aggregate? | Result |
|---|---|---|
| 10.0.0.0/25 + 10.0.0.128/25 | Yes | 10.0.0.0/24 |
| 192.168.0.0/24 + 192.168.1.0/24 | Yes | 192.168.0.0/23 |
| 10.0.0.0/24 + 10.0.2.0/24 | No | Gap between them — not contiguous |
| 10.0.1.0/24 + 10.0.2.0/24 | No | 10.0.1.x does not start on /23 boundary |
Frequently Asked Questions
What is route summarization?
Route summarization (supernetting) combines multiple smaller networks into one larger one by shortening the prefix length. Four consecutive /24s that share the same first 22 bits can be summarized as a single /22, reducing routing table size.
When can two subnets be aggregated?
Two subnets can be aggregated if they are the same size, adjacent (no gap), and together form a block that starts on a power-of-two boundary. 10.0.0.0/25 + 10.0.0.128/25 → 10.0.0.0/24. But 10.0.1.0/24 + 10.0.2.0/24 cannot aggregate because 10.0.1.0 is not on a /23 boundary.
Why does BGP need route aggregation?
Without aggregation, every /24 network worldwide would need its own BGP route entry — hundreds of millions of routes. Aggregation keeps the global table at roughly 1 million entries. ISPs advertise large summary prefixes; cloud providers like AWS advertise superblocks rather than individual customer ranges.
Why it matters for routing
Without route aggregation, every /24 network would need its own entry in global BGP routing tables. The internet's BGP table would have hundreds of millions of entries instead of ~1 million. ISPs and cloud providers perform aggressive route aggregation, advertising only their allocated superblocks (e.g. AWS advertises large /8-/12 prefixes, not millions of individual ranges).
In AWS VPCs, route aggregation lets you simplify Transit Gateway route tables: instead of routing each subnet separately, route the entire VPC CIDR (/16).