Many Internet Service Providers implement a network configuration technique known as carrier-grade NAT or large-scale NAT. Wikipedia describes carrier-grade NAT:
“Carrier-grade NAT (CGN), also known as large-scale NAT (LSN), is an approach to IPv4 network design in which end sites, in particular residential networks, are configured with private network addresses that are translated to public IPv4 addresses by middlebox network address translator devices embedded in the network..."
Carrier-grade NAT, or CGN from here on, solves an IP addressing problem for ISPs, but causes a variety of problems for their customers. These problems are caused by CGN essentially interrupting what should be end-to-end connections, such as those established by Screens when connecting to a remote computer. For further information, see Assessing the Impact of NAT444 on Network Applications.
The following symptoms are associated with CGN-affected Screens connections:
- Upon initiating a Screens connection to the IP address in question, the application will display "Connecting..." but the connection is never established.
- The Screens diagnostics report will include a time-out error.
- The remote IP address will fall within the range of 100.64.0.0 - 100.127.255.255. This block of addresses is set aside for "private" networks, such as ISPs.
You can check whether you’re on a CGN network by visiting a Web site such as ipaddress.com or whatsmyip.com and comparing the IP address listed to the public/WAN IP address assigned to your Internet gateway. To find your gateway’s public IP address, check the status page in the administrative interface. If the two IP addresses differ, you’re likely on a CGN network. If so, you will have trouble establishing Screens connections to the network in question, but it may be possible to request a static IP address from your ISP to avoid connectivity issues.
Tailscale
Some of our users are able to connect by using Tailscale, which is free for personal use. You'll find instructions on how to use Tailscale with Screens here.
I possess a comprehensive understanding of networking concepts, including the intricate details of network address translation (NAT), carrier-grade NAT (CGN), IPv4 and IPv6 addressing, and related challenges and solutions. My knowledge is rooted in extensive data, research, and practical applications related to computer networking and telecommunications.
Concepts Explained:
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Carrier-grade NAT (CGN) / Large-scale NAT (LSN):
- Definition: Carrier-grade NAT is a method used by Internet Service Providers (ISPs) to manage their IPv4 address shortage. In CGN, ISPs assign private IPv4 addresses to end-users, and these addresses are then translated into public IPv4 addresses through middlebox network address translator devices when traffic reaches the public internet.
- Purpose: The primary objective is to allow multiple customers to share a single public IPv4 address, optimizing address utilization.
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IPv4 Addressing:
- Private Address Range: Addresses in the range of 100.64.0.0 to 100.127.255.255 are reserved for CGN. These addresses are not routable on the public internet but are used internally within the ISP's network.
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Challenges with CGN:
- End-to-end Connectivity Issues: CGN can interrupt direct end-to-end connections, leading to complications for certain applications or services that require direct communication between devices.
- Symptoms: As mentioned in the article, issues like time-outs, failed connections, and specific IP address ranges can indicate a CGN-related problem.
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Identifying CGN:
- IP Address Comparison: Users can identify if they are behind a CGN by comparing their device's public IP address with the WAN IP address assigned to their internet gateway. Discrepancies between the two suggest CGN usage.
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Workarounds and Solutions:
- Static IP Address: Some ISPs might offer static public IP addresses as a solution to the challenges posed by CGN, ensuring consistent connectivity for specific applications or services.
- Tailscale: Tailscale is a private network solution that enables secure and direct connections between devices, even across NAT boundaries. It can help users overcome issues related to CGN by facilitating peer-to-peer connections without relying solely on public IP addresses.
In summary, while carrier-grade NAT offers ISPs a viable solution to IPv4 address depletion, it introduces complexities for end-users, particularly concerning direct connectivity and specific application requirements. Solutions like static IP addresses and tools like Tailscale aim to mitigate these challenges, ensuring optimal user experience and network functionality.
