Supernetting & Route Aggregation

What Is Supernetting?

Supernetting (also called route aggregation or route summarization) is the opposite of subnetting. Instead of dividing a network into smaller subnets, you combine multiple contiguous networks into a single larger network prefix.

Before aggregation (4 routes):
  192.168.0.0/24
  192.168.1.0/24
  192.168.2.0/24
  192.168.3.0/24

After aggregation (1 route):
  192.168.0.0/22

Why Aggregate Routes?

• Smaller routing tables — Fewer entries mean faster lookups and less memory usage on routers.
• Reduced BGP updates — Fewer routes advertised to peers means less churn.
• Simplified management — One summarized route is easier to maintain than many specific ones.

How to Calculate a Summary Route

1. Convert all network addresses to binary.
2. Find the common prefix bits shared by all networks.
3. The summary route uses that common prefix with a shorter mask.

192.168.0.0  = 11000000.10101000.00000000.00000000
192.168.1.0  = 11000000.10101000.00000001.00000000
192.168.2.0  = 11000000.10101000.00000010.00000000
192.168.3.0  = 11000000.10101000.00000011.00000000
                                 ^^^^^^^^
Common prefix: 22 bits → 192.168.0.0/22

Requirements for Aggregation

• Networks must be contiguous — no gaps in the address range.
• The number of networks should be a power of 2 (2, 4, 8, 16...).
• The starting network must be aligned to the summary boundary.

Potential Pitfalls

• Black holes — If only 3 of 4 /24s exist but you advertise /22, traffic to the missing /24 gets routed to your network and dropped.
• Over-aggregation — Aggregating non-contiguous blocks requires advertising a larger block than necessary, wasting address space.
• Longest prefix match — More specific routes always win, so internal routing still works even with summary routes at the edge.