Supernetting in Network Layer

Supernetting is the opposite of subnetting. While subnetting splits a large network into smaller subnets, supernetting combines multiple smaller networks with similar prefixes into a larger network (supernet). It reduces routing table size, simplifies routing, and optimizes IP address usage, primarily through route summarization.

Benefits of Supernetting:

• Saves storage space in routing tables
• Reduces the number of routing updates
• Simplifies routing decisions
• Helps manage the growth of routing tables as the Internet expands

Important Points for Supernetting 

• All the Networks should be contiguous. 
• The block size of every network should be equal and must be in form of 2ⁿ. 
• First Network id should be exactly divisible by whole size of supernet. 

Example: Combining Four Class C Networks

Consider Networks:

200.1.0.0,
200.1.1.0,
200.1.2.0,
200.1.3.0

Original Routing Table:

First, let's check whether three conditions are satisfied or not: 

1. Contiguous Networks

• All networks must be next to each other without gaps.
• Example: 200.1.0.0 – 200.1.0.255 is the first network.
• Add 1 to the last IP (200.1.0.255 + 1 = 200.1.1.0) to get the next network.
• Repeat to ensure all networks are contiguous.

2. Equal Network Size

• All networks must have the same number of IP addresses.
• For Class C networks, each has 256 addresses (/24).

3. First IP Aligns with Supernet Size

• Total supernet size = sum of all networks (in powers of 2, e.g., 4 × 256 = 1024 addresses).
• Convert the first IP to binary.
• If the last n bits (where n = bits representing supernet size) are all 0, the first IP is correctly aligned for supernetting.

In the given example first IP is 200.1.0.0 and whole size of supernet is 4*2⁸ = 2¹⁰. If last 10 bits of first IP address are zero then IP will be divisible. 

👁 Supernet example

Last 10 bits of first IP address are zero (highlighted by green color). All conditions are satisfied, so these four networks can be combined into one supernet.

Advantages of Supernetting

• Control and reduce network traffic 
• Helpful to solve the problem of lacking IP addresses 
• Minimizes the routing table i.e, it cannot cover a different area of the network when combined and all the networks should be in the same class and all IP should be contiguous