Public vs Private IP Addresses - IPSeekr Blog

Public vs Private IP Addresses Explained

Published: August 22, 2023

When discussing IP addresses, one of the most important distinctions is between public and private IPs. Though they both serve the purpose of identifying devices on networks, they function in fundamentally different ways. This article will explain the differences, how they work together, and why this dual-address system is essential for modern networks.

The Basic Difference: External vs. Internal

The simplest way to understand the difference between public and private IP addresses is to think of them as external and internal addresses:

  • Public IP address: Your "external" address visible to the entire internet
  • Private IP address: Your "internal" address visible only within your local network

This is similar to how a business might have one public street address (public IP) but many internal office numbers (private IPs) to direct visitors once they're inside the building.

Public IP Addresses: Your Internet Identity

What is a Public IP Address?

A public IP address (sometimes called an external or global IP) is a unique numerical identifier assigned to your device or network by your Internet Service Provider (ISP). It's visible to the outside world and allows your device to communicate with servers, websites, and other devices across the internet.

Characteristics of Public IP Addresses:

  • Globally unique: No two active devices on the internet can have the same public IP address simultaneously
  • Assigned by ISPs: Your service provider allocates these addresses from their assigned blocks
  • Internet-routable: Can be reached from anywhere on the internet
  • Limited resource: Particularly for IPv4, public addresses are a scarce commodity

Types of Public IP Addresses:

Public IPs can be further categorized as:

  • Dynamic public IPs: Change periodically (typically when your router reboots or after a specific lease period expires)
  • Static public IPs: Remain constant and are typically used for services that require consistent addressing

Private IP Addresses: Your Local Network Identity

What is a Private IP Address?

A private IP address is used to identify devices within a local network. These addresses are not directly accessible from the internet and can be reused in different private networks worldwide. When you connect multiple devices in your home to your Wi-Fi router, each gets its own private IP address.

Reserved Private IP Ranges

The Internet Assigned Numbers Authority (IANA) has reserved specific ranges of IPv4 addresses for private networks:

  • Class A: 10.0.0.0 to 10.255.255.255 (10.0.0.0/8)
  • Class B: 172.16.0.0 to 172.31.255.255 (172.16.0.0/12)
  • Class C: 192.168.0.0 to 192.168.255.255 (192.168.0.0/16)

For IPv6, private addresses typically use the fd00::/8 prefix, known as Unique Local Addresses (ULAs).

Characteristics of Private IP Addresses:

  • Locally unique: Must be unique within your network but can be duplicated in other networks
  • Non-routable: Cannot be directly accessed from the internet
  • Assigned locally: Typically assigned by your router's DHCP server
  • Free resource: No cost associated with using these addresses

How Public and Private IPs Work Together: The Role of NAT

If private IPs can't be accessed from the internet, how do multiple devices in your home access the internet simultaneously? The answer lies in Network Address Translation (NAT).

Network Address Translation (NAT)

NAT is a process that routers use to translate between private and public IP addresses. Here's how it works:

  1. When a device on your private network sends a request to the internet, the router records which internal device made the request
  2. The router replaces the private source IP with its public IP address
  3. When responses come back, the router uses its tracking table to determine which private IP address should receive the incoming data
  4. The router forwards the response to the correct device on the private network

This process allows multiple devices with private IPs to share a single public IP address. NAT has become essential due to the scarcity of IPv4 addresses and also provides a layer of security by hiding your internal network structure.

Port Address Translation (PAT)

Modern home networks typically use a more sophisticated form of NAT called Port Address Translation (PAT) or NAT overload. This allows the router to keep track of different connections from different devices by assigning unique port numbers to each connection. This is why dozens of devices in your home can all use the internet simultaneously through a single public IP address.

Advantages of the Dual IP Address System

The use of both public and private IP addresses offers several important benefits:

Conservation of IPv4 Addresses

With only about 4.3 billion possible IPv4 addresses and over 10 billion internet-connected devices worldwide, private addressing with NAT has been crucial in extending the usable life of IPv4.

Enhanced Security

Private IP addressing adds a layer of security since devices aren't directly accessible from the internet. Your router essentially acts as a gatekeeper, preventing direct unsolicited connections from the outside world to your devices.

Network Organization

Private addressing allows for easier network management and organization, including the creation of subnets and logical network divisions.

How to Find Your IP Addresses

Finding Your Public IP

You can find your public IP address by:

  • Using IPSeekr's homepage tool
  • Searching "what is my IP" on search engines like Google
  • Visiting other IP lookup services

Finding Your Private IP

To find your device's private IP address:

  • On Windows: Open Command Prompt and type "ipconfig"
  • On macOS: Go to System Preferences > Network > Select your connection > Details
  • On iOS: Go to Settings > Wi-Fi > Tap the info icon (i) next to your network
  • On Android: Go to Settings > Network & Internet > Wi-Fi > Select your network
  • On Linux: Open Terminal and type "ip addr show" or "ifconfig"

Common Scenarios and Use Cases

Example: A Typical Home Network

In a typical home setup:

  • Your ISP assigns one public IP address to your router (e.g., 203.0.113.42)
  • Your router assigns private IP addresses to your devices:
    • Laptop: 192.168.1.2
    • Smartphone: 192.168.1.3
    • Smart TV: 192.168.1.4
    • Gaming console: 192.168.1.5
  • All these devices communicate with the internet through the single public IP address

Port Forwarding

Sometimes you may want external access to a specific device on your private network (like a security camera or a game server). This is accomplished through port forwarding, where your router is configured to direct specific incoming traffic to a particular device on your private network.

Public and Private IPs in IPv6

The distinction between public and private addresses is somewhat different in IPv6 due to its vast address space:

  • Global Unicast Addresses (GUA): Equivalent to public IPv4 addresses
  • Unique Local Addresses (ULA): Similar to private IPv4 addresses, starting with fd00::/8
  • Link-Local Addresses: Automatically configured addresses beginning with fe80::/10, used for communication on a single network segment

With IPv6, NAT is technically unnecessary due to the abundance of addresses, but many networks still implement similar concepts for security and easier network management.

Conclusion

Understanding the difference between public and private IP addresses is fundamental to grasping how modern networks function. The public/private IP system, combined with NAT, has allowed the internet to scale far beyond what the original IPv4 addressing scheme could support on its own.

As the internet continues to evolve with IPv6 adoption, the concepts of address scoping and network boundaries remain important for security and organization. Whether you're troubleshooting connection issues, setting up a home network, or just curious about how your devices communicate, the public/private IP distinction is an essential concept to understand.

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