Difference Between Public, Private IPv4 Addresses and the Carrier-Grade NAT Range

The IPv4 protocol shortage and the need for classification

When the internet was designed in past decades, the IPv4 protocol used 32-bit addresses, creating a theoretical ceiling of approximately 4.3 billion unique addresses. At the time, this seemed like an astronomical number impossible to exhaust. However, with the proliferation of computers, smartphones, servers, and internet-connected IoT devices, this addressing space collapsed. To manage this scarce resource, network engineering divided the universe of addresses into two main categories: public and private, later introducing intermediate sharing mechanisms.

In practical terms, understanding this division prevents configuration failures in corporate and residential networks. An IP address essentially functions like a house's postal address: without it, data packets do not know where to go. However, not all addresses need to circulate freely through the global mail system. This is precisely where the fundamental distinctions that shape modern internet infrastructure come into play.

Public IPv4 addresses and global routability

A public IP address is visible and routable across the entire global internet. Think of it as your home's official address registered with the city, accessible by any delivery service on the planet. When you visit a news website or host a cloud application, the destination server needs to see your real public address to return the requested responses.

These addresses are managed globally by IANA (Internet Assigned Numbers Authority) and distributed regionally by entities like ARIN or NIC.br. Because the global stock was depleted years ago, acquiring an additional public IPv4 address has become a bureaucratic and costly process. Internet service providers and large corporations must rent or buy these blocks on the secondary market, which drastically increased the operational cost of digital infrastructure.

Private IPv4 addresses and local network isolation

To bypass the premature exhaustion of public IPs, engineering established RFC 1918, defining specific ranges of IP addresses strictly intended for internal use. In practice, a private address functions like an office extension or an apartment number in a residential building: it only makes sense within that closed community. Outside of that scope, the number has no value and cannot be routed directly by the internet backbone.

The reserved ranges comprise three major blocks known to network administrators: class A (10.0.0.0 to 10.255.255.255), class B (172.16.0.0 to 172.31.255.255), and class C (192.168.0.0 to 192.168.255.255). Any home or corporate router uses these addresses to organize computers, printers, and phones connected via Wi-Fi or cable. For these devices to access the internet, NAT (Network Address Translation) steps in, translating the private IP into a valid public IP before sending the packet outward.

Growth bottlenecks and the arrival of Carrier-Grade NAT

Even with the widespread use of private addresses and domestic NAT, the number of new broadband subscribers continued to grow exponentially. Internet service providers (ISPs) began running out of sufficient public IPv4 addresses to deliver a dedicated IP to every residential customer. The solution found by network engineering to postpone collapse was Carrier-Grade NAT, also known as CGNAT or carrier-scale NAT.

In practice, CGNAT operates like traditional NAT, but runs on the ISP's core equipment rather than your home router. The provider places hundreds or even thousands of clients behind a single shared public IP address. For this to work without massive conflicts, the operator's equipment manipulates not only IP addresses but also the network ports of each connection, creating a complex digital labyrinth.

Operational impacts and limitations of CGNAT

Although CGNAT saved telecommunications operators from immediate collapse due to a lack of addresses, it introduces considerable technical disadvantages for the end user. Since multiple clients share the same public IP, port mapping is no longer straightforward. In practice, this means online games requiring open NAT types, home security camera servers, remote access via VPN, and peer-to-peer connections face severe performance and functionality failures.

Furthermore, CGNAT directly impacts cybersecurity investigations and security systems. When dozens of users share the same public IP, connection logs on an external server show only the ISP's IP. To identify which specific client performed a certain action, authorities must request internal port mapping logs from the internet provider, adding bureaucracy and slowness to proceedings.

Final considerations on the transition to the future

The division between public, private, and CGNAT ranges reflects human ingenuity in extending the lifespan of a technology that surpassed all original scale predictions. However, CGNAT is a technical palliative rather than a permanent solution. Excessive port sharing creates performance bottlenecks and breaks fundamental principles of the original internet architecture, which envisioned end-to-end connectivity without translator intermediaries.

The only robust and technically sound way forward for this scenario is the accelerated and universal adoption of the IPv6 protocol. With an immensely superior address space, IPv6 permanently eliminates the need for NAT and returns to each device its own unique identity on the global network. Until this transition is complete, understanding the nuances between public, private, and CGNAT will remain an essential requirement for any technology professional.