Subnetting 101

Step 10 — Supernetting & Route Summarization

Supernetting is the reverse of subnetting: combining multiple smaller networks into one larger network. This is essential for route summarization, which reduces routing table size and improves network efficiency.

Why Supernetting Matters

Without summarization (bad):

Routing table has 256 entries:
192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
... (253 more routes)

With summarization (good):

Single route covers all 256 networks:
192.168.0.0/16

How Supernetting Works

To combine networks, you need contiguous address blocks that share common high-order bits. You then reduce the prefix length to encompass all networks.

Example: Combine 4 /24 networks

192.168.0.0/24 → 11000000.10101000.00000000.xxxxxxxx

192.168.1.0/24 → 11000000.10101000.00000001.xxxxxxxx

192.168.2.0/24 → 11000000.10101000.00000010.xxxxxxxx

192.168.3.0/24 → 11000000.10101000.00000011.xxxxxxxx

Common bits: first 22 bits are identical

Summary: 192.168.0.0/22

Step-by-Step Process

  1. List all networks to be summarized
  2. Convert to binary and align them
  3. Find the common bits from left to right
  4. Count the common bits — this is your new prefix
  5. Zero out the remaining bits for the summary network

Requirements for Valid Summarization

1. Networks must be contiguous

You can summarize 192.168.0.0, 192.168.1.0, 192.168.2.0, 192.168.3.0
You cannot summarize 192.168.0.0 and 192.168.5.0 (gap in the middle)

2. Number of networks must be a power of 2

You can summarize 2, 4, 8, 16, 32... networks
You cannot perfectly summarize 3 or 5 networks

3. First network must start on a proper boundary

To summarize 4 /24s into a /22, the first /24 must be at .0, .4, .8, etc.

Worked Examples

Example 1: Summarize 8 /24 networks

Networks: 10.10.0.0/24 through 10.10.7.0/24

10.10.0.0 → third octet: 00000000

10.10.1.0 → third octet: 00000001

10.10.2.0 → third octet: 00000010

10.10.3.0 → third octet: 00000011

10.10.4.0 → third octet: 00000100

10.10.5.0 → third octet: 00000101

10.10.6.0 → third octet: 00000110

10.10.7.0 → third octet: 00000111

Common bits in third octet: 5 bits (00000xxx)
Total prefix: 8 + 8 + 5 = 21

Summary: 10.10.0.0/21

Example 2: Summarize non-contiguous (tricky)

Networks: 172.16.32.0/24 through 172.16.47.0/24 (16 networks)

First network: 172.16.32.0 (third octet = 32 = 00100000)

Last network: 172.16.47.0 (third octet = 47 = 00101111)

Common bits in third octet: 4 (0010xxxx)

Summary: 172.16.32.0/20

Quick Mental Math Method

Formula:

New prefix = Original prefix - log₂(number of networks)

Example: Summarize 8 /24 networks

log₂(8) = 3

New prefix = 24 - 3 = /21

Example: Summarize 4 /27 networks

log₂(4) = 2

New prefix = 27 - 2 = /25

Practice Problems

  1. Summarize: 192.168.16.0/24 through 192.168.31.0/24

  2. Summarize: 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24, 10.0.3.0/24

  3. Can you summarize 172.16.1.0/24 and 172.16.3.0/24? Why or why not?

  4. What's the summary route for 10.100.0.0/16 through 10.103.0.0/16?

Show Solutions

1. 192.168.16.0/24 - 192.168.31.0/24

16 networks, log₂(16) = 4, so 24 - 4 = /20

Summary: 192.168.16.0/20

2. 10.0.0.0/24 - 10.0.3.0/24

4 networks, log₂(4) = 2, so 24 - 2 = /22

Summary: 10.0.0.0/22

3. 172.16.1.0/24 and 172.16.3.0/24

Cannot be perfectly summarized!

They're not contiguous (missing .2.0). A /22 summary would include .0, .1, .2, .3 — wasting addresses.

4. 10.100.0.0/16 - 10.103.0.0/16

4 networks, log₂(4) = 2, so 16 - 2 = /14

Summary: 10.100.0.0/14

Real-World Application

BGP Route Aggregation

ISPs use supernetting to advertise single routes instead of thousands. An ISP with the block 203.0.113.0/24 through 203.0.127.0/24 (15 networks) might advertise 203.0.112.0/20 (covers 16 /24s, slightly oversized).

Enterprise WAN

A branch office with subnets 10.50.0.0/24, 10.50.1.0/24, 10.50.2.0/24, 10.50.3.0/24 can advertise a single 10.50.0.0/22 route to headquarters.

Checkpoint

Before moving on, make sure you can:

  • Explain why route summarization improves network performance
  • Identify when networks can and cannot be summarized
  • Calculate summary routes using binary or the quick formula
  • Apply summarization concepts to real routing scenarios