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1. Overview

More often than never, as network administrators, we commonly come across the terms such as public, non-routable addresses while designing and troubleshooting. This is because these addresses form the backbone of the clustered and complex computing network – the internet.

In this tutorial, we’ll elucidate on a few why’s and how’s such as why to use non-routable IP addresses, how non-routable IP with NAT helps in expanding the IPv4 internet and, how it provides interim support for IPv6 migration.

2. What Is an IP Address?

Generally, humans identify and communicate with each other using their names.

Quite analogous to this fact, machines use IP addresses to identify and interact with each other in the computing world. Therefore, any device connected within the network must have a unique IP address to communicate with other devices.

Today, the internet exists because of this unique addressing technique. The IANA manages the IP address ranges for networks that connect to the internet. IP Version 4 and 6 are the two different IP addressing schema available. The former is traditional and oft-used addressing schema in 32 bits with dotted-decimal notation, while the latter is modern, newly adopted with 128 bits in hexadecimal separated by a colon.

3. Public vs. Private IP Address

IPv4 addressing is of two types: public and private addresses.

Public IP addresses are the basic constructs for a long chain of globally connected devices on the internet. They are unique, routed globally through border gateway protocol, and are called the global or WAN address.

On the other hand, private IP addresses do not rout on the internet. Hence, they are also known as non-routable IP addresses. They help the organization create its local network without exposing its routes to the internet. However, troubleshooting and maintenance of private networks are complex compared to public networks.

4. Non-routable IP Address

Billions of people are on the internet, and its growth is exponential. With a maximum of 4 billion available addresses, the size of IPv4 address space is fast depleting. Even though IPv6 has umpteen advantages, due to many practical reasons, it cannot supplant IPv4. Therefore, both versions must co-exist for some time for seamless migration.

However, RFC 1918 was released in 1996 to counter the IPv4 depletion. It quotes that “there can be networks within an autonomous system known as Non-Routable Address Space. These defined IP ranges can build the private or local network”.

There are three different IP ranges defined in RFC 1918. They are:

  • ( Range: – ) – Available IPs: 16,777,214
  • ( Range: – ) – Available IPs: 1,048,574
  • ( Range: – ) – Available IPs: 65,534

Further on, these private networks connect to the internet through NAT technology which brings us to the next topic.

5. Network Address Translation (NAT)

Due to the internet growth spurt, there is an increase in the usage of business and home networks which, in turn, has led to IPv4 address space depletion. The obvious solution is to restructure the entire internet infrastructure with IPv6, which might take several years to implement.

The NAT (RFC 1631) comes to the rescue and fills this glaring gap.

NAT acts as a middleman between the internet and private networks. It works at layer 3 of the OSI model and operates on a router. It enables a private network that uses the non-routable IP addresses to connect with the internet through the translated Public IP. Therefore, it advertises only one IP address for the entire private network to the outside world.

In this way, it strengthens the network security, and in the process, conserves the IPv4 address space. Home broadband network is the best example of this kind.

6. Internet

A network is a group of computer devices connected through a cable. With the help of IP protocols and addresses, they know each other, update information, and exchange data.

The internet [interconnected networks] is an extensive collection of computing devices connected across the globe. Packets and protocols are the bloodlines of the internet. Here, packets contain information about the source, destination, application ports, and data, whereas the protocols determine the path to reach the destination through IP addresses.

6.1. Internet Traffic Flow – Walkthrough

The below illustration explains how a typical home or enterprise device communicates with the services on the internet:

How Internet Works?

To begin with, we have two networks built with non-routable private addresses. These private home networks have a IP segment, wherein the computers and other communication gadgets get one IP for connection. The traffic from all these devices gets terminated in the gateway.

Further on, the NAT configured on the router, with all incoming traffic from segments, gets converted into Public IP. Subsequently, when the traffic reaches Google (or any other destination), it identifies that the incoming traffic is from the (Source IP address), not from the (non-routable private IPs).

Along similar lines, the private enterprise network has the same traffic flow. It uses /8 segment to accommodate the computing machines in offices and data centers with a minor change in IP segment space. Subsequently, non-routable segment mapped to public IP addresses for internet communication.

Likewise, the traffic reaching Google (or any other destination) identifies as the packet’s source IP address and, in this way, conceals the existence of underlying non-routable private networks.

This feature bolsters the network security of an organization. Depending on the business requirements, the app-aware routing policies enable the packet priorities based on the traffic classifications.

In this case, 254 usable IP addresses of the private home network are NATed with one public IP address. Meanwhile, a single public IP address manages 16 lakh addresses in the private enterprise network. Clearly, there is a multi-fold saving in the usage of Public IP addresses.

7. Conclusion

In this article, we have learned that the IP address reserved for building the private networks is non-routable. This concept evolved to prevent the exhaustion of the IPv4 address space. IPv6 is the future of the internet, but the migration of the entire internet infrastructure into IPv6 may not be feasible in a short time. For this, non-routable addresses with NAT serves as an interim solution to continue in IPv4 and provide some breathing time for IPv6-based internet infrastructure.

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