Route Summarization / Supernetting

Learn how to aggregate multiple networks into a single route for more efficient routing tables.

What is Supernetting?

Supernetting (also called route aggregation or summarization) combines multiple smaller networks into one larger network announcement. Instead of advertising many routes, you advertise just one route that covers them all. This reduces routing table size and makes networks more manageable.

Main Goal
Reduce the number of routes in routing tables while maintaining connectivity to all networks.

Requirements for Summarization

Two main requirements must be met: 1. **Contiguous Address Space**: The networks must be adjacent to each other 2. **Power of Two**: You need exactly 2, 4, 8, 16, etc. networks to summarize cleanly If you have 3 or 5 networks, you cannot create a perfect summary without including unused space.

Summarization Examples

Basic Example: Two /24s
Individual Networks:
192.168.0.0/24
192.168.1.0/24
↓ Summarizes to ↓
192.168.0.0/23
Two adjacent /24 networks combine into one /23 - 512 total addresses (2 × 256)
Four /24s
Individual Networks:
10.1.0.0/24
10.1.1.0/24
10.1.2.0/24
10.1.3.0/24
↓ Summarizes to ↓
10.1.0.0/22
Four adjacent /24 networks combine into one /22 - 1,024 total addresses (4 × 256)
Eight /27s
Individual Networks:
172.16.0.0/27
172.16.0.32/27
172.16.0.64/27
172.16.0.96/27
172.16.0.128/27
172.16.0.160/27
172.16.0.192/27
172.16.0.224/27
↓ Summarizes to ↓
172.16.0.0/24
Eight adjacent /27 networks combine into one /24 - 256 total addresses (8 × 32)

Step-by-Step Process

  1. List all networks you want to summarize
  2. Convert network addresses to binary
  3. Find the common bits from left to right
  4. Count the common bits to get your new prefix length
  5. Verify the summary covers all your networks
  6. Check if any unwanted networks are included

Binary Example

Summarizing 192.168.0.0/24 and 192.168.1.0/24

NetworkBinary Representation
192.168.0.011000000.10101000.00000000.00000000
192.168.1.011000000.10101000.00000001.00000000
Analysis
First 23 bits are identical, so summary is 192.168.0.0/23

Benefits of Route Summarization

  • Smaller routing tables mean faster lookups
  • Reduced memory usage on routers
  • Less routing protocol overhead
  • Simpler network management
  • Better network stability (fewer route changes)

Common Pitfalls

Including Unwanted Networks

Problem: Summary might include networks you don't own or want to advertise

Example: Summarizing 192.168.0.0/24 and 192.168.2.0/24 as 192.168.0.0/22 also includes 192.168.1.0/24 and 192.168.3.0/24

Non-Contiguous Networks

Problem: Trying to summarize networks that aren't adjacent

Example: Cannot cleanly summarize 10.1.0.0/24 and 10.1.5.0/24 without including unwanted space

Wrong Powers of Two

Problem: Having 3, 5, 6, 7 networks instead of 2, 4, 8, 16

Example: Three /24s cannot be perfectly summarized without wasting addresses

Quick Reference Table

Input NetworksSummary PrefixRoutes Saved
2 × /24/231 route
4 × /24/223 routes
8 × /24/217 routes
16 × /24/2015 routes
2 × /23/221 route
4 × /23/213 routes