Same Public IP Address: Can Multiple People Share One?

Understanding Shared Public IP Addresses

Typically, most internet users assume they possess a unique public IP address. However, situations arise where this isn't the case. This can lead to confusion regarding network connectivity and online identification.

The Scenario: Multiple Users, One IP

What happens when multiple users appear to be sharing the same IP address? This is a common occurrence, and understanding the reasons behind it is crucial for troubleshooting network issues.

Today’s featured question originates from SuperUser, a question-and-answer platform that is part of the Stack Exchange network.

How IP Address Sharing Occurs

Several factors can contribute to multiple devices sharing a single public IP address. These include:

• Network Address Translation (NAT): This is the most frequent cause. NAT allows numerous devices on a private network to share a single public IP address.
• Carrier-Grade NAT (CGNAT): Internet Service Providers (ISPs) utilize CGNAT to manage their IPv4 address space, especially as IPv4 addresses become scarce.
• Large Organizations: Businesses and institutions often employ NAT to connect many internal devices to the internet using a limited number of public IPs.

Essentially, a private network uses internal, non-routable IP addresses. When data needs to travel to the public internet, NAT translates these private addresses to the single public IP assigned to the network.

SuperUser's Role in Addressing Tech Questions

The original inquiry was posed and answered on SuperUser, a community-focused platform dedicated to providing solutions for technical questions. It’s a valuable resource for users seeking assistance with computing and internet-related problems.

The accompanying image is credited to Nicolas Nova, sourced from Flickr.

Understanding Shared Public IP Addresses

A SuperUser user, Spartan, posed a question regarding the possibility of multiple individuals sharing the same public IP address. The core of the inquiry centers around whether distinct connections, even through the same Internet Service Provider (ISP), can result in identical public IP assignments.

The Standard Scenario

Typically, two computers cannot possess the same public, or external, IP address unless they are connected through a shared router. When utilizing a single router, multiple devices can indeed share the same public IP address while maintaining unique private, or local, IP addresses within the network.

Spartan's Encounter

Spartan and a friend, both subscribers to the same ISP with separate accounts and connections, discovered they were assigned the same public IP address. This was confirmed using Google's IP lookup tool, displaying 112.133.229.29 for both users. Their respective routers reported internal IP addresses of 10.1.102.93 and 10.1.101.29.

Implications of a Non-Unique IP

Spartan noted that running an Apache web server on their computer was inaccessible via the shared public IP address (http://112.133.229.29/index.html). However, the friend could access the server using the router's local address (http://10.1.102.93/index.html). This led Spartan to suspect a shared DNS configuration within their ISP’s network.

Access Restrictions

Attempts by a third party, connected through a different ISP, to access Spartan’s computer using the router-based link (http://10.1.102.93/index.html) were unsuccessful. This further highlighted the limitations of accessing devices via local IP addresses from outside the network.

The Core Question Revisited

Spartan questioned how their ISP could implement such a system, given that network requests are typically identified and routed based on the originating public IP address. The fundamental question remains: is it genuinely possible for different users to share the same public IP address, and if so, how is this achieved?

Possible Explanations

• NAT (Network Address Translation): ISPs commonly use NAT to allow multiple customers to share a single public IP address.
• CGNAT (Carrier-Grade NAT): A more extensive form of NAT used by ISPs to manage a large pool of customers with a limited number of public IP addresses.
• Dynamic IP Allocation: IP addresses are often assigned dynamically, and it's possible, though less common, for the same address to be assigned to different customers at different times.

The situation Spartan describes is most likely due to the ISP utilizing CGNAT, a practice becoming increasingly common as the availability of IPv4 addresses diminishes.

Understanding Shared Public IP Addresses

A SuperUser community member, Gestudio Cloud, provides insight into why you might be sharing a public IP address.

Network Address Translation (NAT) in Action

As DavidPostill previously pointed out, your Internet Service Provider (ISP) employs NAT on their network devices before your data reaches the wider internet.

Essentially, you and other customers within your ISP’s service area are connected within a large-scale Metropolitan Area Network (MAN). This functions similarly to how your home router establishes a Local Area Network (LAN), but operates on a significantly broader scale.

The Rationale Behind ISP Practices

Why do ISPs utilize this approach? The reason is straightforward: they aim to conserve the number of publicly available IPv4 addresses.

This is often a necessity, as the number of customers can exceed the quantity of available public IPv4 addresses.

IPv4 Address Depletion

The global pool of unassigned IPv4 addresses has been depleted for several years.

Consequently, growing ISPs are unable to acquire new IPv4 subnets unless they purchase them from other ISPs that are ceasing operations.

The IPv6 Solution

The long-term solution lies in the adoption of IPv6 addresses.

However, transitioning to IPv6 requires hardware upgrades, configuration changes, and substantial investment of both money and time, making the implementation of a large MAN a more expedient option for ISPs.

Requesting a Dedicated IPv4 Address

You can contact your ISP to inquire about obtaining a dedicated IPv4 address.

Be prepared for the possibility of an additional charge for this service, as it provides a unique IPv4 address specifically for your computer or location.

Do you have additional information to contribute to this explanation? Share your thoughts in the comments section below.

For further insights from other knowledgeable Stack Exchange users, explore the complete discussion thread here.