ad placeholder image ad placeholder image

Carrier-Grade NAT (CGNAT): Complete Guide

Carrier-Grade NAT (CGNAT), also known as Large-Scale NAT (LSN) or NAT444, is a network address translation method used by Internet Service Providers to share a single public IP address among multiple customers. Understanding CGNAT is crucial for troubleshooting connectivity issues and understanding modern internet infrastructure.

What is CGNAT?

CGNAT is Network Address Translation performed at the ISP level, adding an additional layer of NAT between your home router and the internet. This allows ISPs to conserve IPv4 addresses by sharing one public IP among many customers.

The Problem CGNAT Solves

IPv4 address exhaustion: - Only 4.3 billion IPv4 addresses exist - Billions of internet users and devices - ISPs running out of public IPs - New customers need connectivity

Traditional solution: Each customer gets one public IP Customer uses NAT at home for multiple devices ISP needs one public IP per customer

CGNAT solution: Multiple customers share one public IP ISP uses NAT at their level Customers get private IPs from ISP Conserves public IPv4 addresses

How CGNAT Works

Double NAT Architecture

Without CGNAT (traditional): Your Device (192.168.1.100) ↓ Home Router NAT (Public IP: 203.0.113.45) ↓ Internet

With CGNAT (NAT444): Your Device (192.168.1.100) ↓ Home Router NAT (ISP Private IP: 100.64.0.50) ↓ ISP CGNAT (Public IP: 203.0.113.45 - shared) ↓ Internet

The Translation Process

Step 1: Your device to home router ``` Source: 192.168.1.100:54321 Destination: 93.184.216.34:80

Home router translates: Source: 100.64.0.50:12345 Destination: 93.184.216.34:80 ```

Step 2: Home router to ISP CGNAT ``` Source: 100.64.0.50:12345 Destination: 93.184.216.34:80

ISP CGNAT translates: Source: 203.0.113.45:65432 Destination: 93.184.216.34:80 ```

Step 3: Website sees Source: 203.0.113.45:65432 (Shared with many other customers)

RFC 6598 Address Space

CGNAT uses special private range: Range: 100.64.0.0 to 100.127.255.255 CIDR: 100.64.0.0/10 Purpose: Shared Address Space for CGNAT Total addresses: 4,194,304

Why this range? - Not used by home networks - Separate from RFC 1918 (192.168.x.x, 10.x.x.x, 172.16.x.x) - Prevents conflicts - Specifically for ISP use

Detecting CGNAT

Check Your WAN IP

Method 1: Router admin page 1. Log into router (192.168.1.1) 2. Check WAN/Internet IP address 3. If it starts with 100.64.x.x → CGNAT 4. If it's a public IP → No CGNAT

Method 2: Compare IPs 1. Check router's WAN IP 2. Visit whatismyip.com or ippigly.com 3. If different → CGNAT 4. If same → No CGNAT

Example: Router WAN IP: 100.64.25.100 (private) Public IP (from website): 203.0.113.45 Result: You're behind CGNAT

Signs You're Behind CGNAT

Indicators: - Router WAN IP is 100.64.x.x - Port forwarding doesn't work - Can't host servers - Gaming NAT type: Strict/Moderate - VoIP quality issues - P2P applications struggle

Problems Caused by CGNAT

Port Forwarding Impossible

Traditional NAT: Internet → Your Public IP:80 → Your Server Port forwarding works

With CGNAT: Internet → Shared Public IP:80 → ??? ISP's CGNAT doesn't know which customer Port forwarding impossible from your end

Impact: - Can't host web servers - Can't host game servers - Can't access security cameras remotely - Can't run services from home

Gaming Issues

NAT Types: Open NAT: Best (no CGNAT) Moderate NAT: Okay (single NAT) Strict NAT: Poor (CGNAT)

Problems: - Can't host game lobbies - Reduced matchmaking pool - Connection issues - Voice chat problems - Longer wait times

Affected games: - Call of Duty - Fortnite - Minecraft (hosting) - Any P2P games

VoIP and Video Calling

Issues: - Poor call quality - Connection failures - One-way audio - Dropped calls - Increased latency

Affected services: - Skype - Zoom (sometimes) - WhatsApp calls - FaceTime - SIP phones

P2P Applications

Affected: - BitTorrent (reduced peers) - File sharing - Video conferencing - Remote desktop - Cryptocurrency nodes

Why: - Can't accept incoming connections - Rely on relay servers - Reduced performance - Limited functionality

Multiple Devices Issues

Problem: ISP CGNAT has limited ports per customer Many devices sharing those ports Port exhaustion possible

Symptoms: - Intermittent connectivity - Some devices can't connect - Random disconnections - Slow performance

IP-Based Services

Problems: - Shared IP with other customers - IP reputation issues - Geolocation inaccuracy - Can't whitelist your IP - Blacklist affects multiple users

Example: One customer's abuse → IP blacklisted All customers sharing that IP affected Email delivery issues Website access blocked

Workarounds and Solutions

Request Public IP from ISP

Best solution: Contact ISP Request dedicated public IPv4 address May require: - Business plan upgrade - Additional monthly fee ($5-20) - Static IP service

Benefits: - Eliminates CGNAT - Port forwarding works - Gaming NAT: Open - Full functionality restored

Use IPv6

If ISP provides IPv6: Enable IPv6 on router Get public IPv6 address No NAT needed End-to-end connectivity

Benefits: - No CGNAT - Direct connectivity - Port forwarding works - Future-proof

Limitations: - Not all services support IPv6 - Some devices don't support IPv6 - Dual-stack still needed

VPN with Port Forwarding

Some VPN providers offer port forwarding: Your Network → VPN → Internet VPN provides public IP and port Forward traffic through VPN

Providers: - PIA (Private Internet Access) - AirVPN - Mullvad

Use cases: - Hosting services - Gaming - Torrenting - Remote access

Cloud/VPS Reverse Proxy

Setup: Rent VPS with public IP Configure reverse proxy Tunnel traffic to your home VPS forwards to your services

Tools: - Nginx reverse proxy - Cloudflare Tunnel - ngrok - frp (fast reverse proxy)

Example with Cloudflare Tunnel: Your Server → Cloudflare Tunnel → Internet No port forwarding needed Works behind CGNAT Free for personal use

UPnP/NAT-PMP (Limited)

May help with some applications: Enable UPnP on router Applications auto-configure NAT Only works for home router NAT Doesn't solve CGNAT

Helps with: - Some games - Torrent clients - Media servers

Doesn't help with: - Hosting public services - Incoming connections through CGNAT

Gaming-Specific Solutions

Use game-specific relays: - Xbox Live relay servers - PlayStation Network - Steam networking - Discord voice servers

Enable DMZ on router: Puts one device outside home NAT Doesn't solve CGNAT May improve gaming slightly Security risk

ISP Perspectives on CGNAT

Why ISPs Use CGNAT

Address conservation: ``` Without CGNAT: 1,000 customers = 1,000 public IPs needed

With CGNAT: 1,000 customers = 10-50 public IPs needed Massive savings ```

Cost savings: - IPv4 addresses cost $20-50 each - Thousands of customers - Millions in savings - Delays IPv6 investment

Rapid deployment: - Quick to implement - No customer equipment changes - Transparent to most users - Solves immediate problem

ISP CGNAT Implementations

Typical ratios: Conservative: 20-50 customers per IP Moderate: 50-100 customers per IP Aggressive: 100-500 customers per IP

Port allocation: Per customer: 256-2048 ports Depends on ISP policy Affects number of simultaneous connections

CGNAT and IPv6 Transition

The Long-Term Solution

IPv6 eliminates need for CGNAT: IPv6 addresses: 340 undecillion Every device gets unique address No NAT needed End-to-end connectivity

Current state: - Many ISPs deploying IPv6 - Dual-stack (IPv4 + IPv6) common - CGNAT for IPv4, native IPv6 - Gradual transition

Dual-Stack Approach

Best practice: IPv6: Native, no NAT IPv4: CGNAT (for legacy) Devices prefer IPv6 Fall back to IPv4 if needed

Benefits: - IPv6 for modern services - IPv4 compatibility maintained - Smooth transition - Best of both worlds

Testing and Troubleshooting

Verify CGNAT Status

Test 1: Check WAN IP ```bash

On router or via admin page

If 100.64.x.x → CGNAT

```

Test 2: Port test 1. Set up port forward on router 2. Test from external network 3. If fails → likely CGNAT

Test 3: Trace route ```bash traceroute 8.8.8.8

Look for 100.64.x.x addresses

Indicates CGNAT

```

Diagnosing CGNAT Issues

Gaming NAT type test: Xbox: Settings → Network → NAT Type PlayStation: Settings → Network → Connection Status PC games: Usually in network settings

Port forwarding test: 1. Configure port forward 2. Use canyouseeme.org 3. Test your port 4. If fails → CGNAT or firewall

Connection test: Try hosting a service Test from external network If unreachable → CGNAT blocking

Best Practices

For Users Behind CGNAT

1. Understand limitations - Know what won't work - Plan accordingly - Don't blame your router

2. Request public IP if needed - Contact ISP - Explain use case - Be prepared to pay

3. Use workarounds - VPN with port forwarding - Cloud services - IPv6 when available

4. Enable IPv6 - Check ISP support - Enable on router - Test connectivity - Prefer IPv6 services

For ISPs

1. Communicate clearly - Inform customers about CGNAT - Explain limitations - Offer alternatives

2. Provide options - Public IP upgrade path - IPv6 deployment - Reasonable pricing

3. Allocate sufficient ports - Don't over-subscribe - Monitor usage - Adjust as needed

4. Deploy IPv6 - Long-term solution - Reduces CGNAT dependency - Better customer experience

Future of CGNAT

Temporary Solution

CGNAT is a stopgap: - Delays IPv4 exhaustion impact - Not a permanent solution - Adds complexity - Degrades user experience

IPv6 Adoption

As IPv6 grows: - CGNAT becomes less necessary - IPv4 relegated to legacy - Better end-to-end connectivity - Improved functionality

Timeline: - Short term (2024-2026): CGNAT prevalent - Medium term (2026-2030): IPv6 majority - Long term (2030+): IPv4 legacy, IPv6 standard

Conclusion

Carrier-Grade NAT is a necessary but imperfect solution to IPv4 address exhaustion. While it allows ISPs to conserve addresses and serve more customers, it introduces limitations that affect gaming, hosting services, and certain applications. Understanding CGNAT helps you diagnose issues, find workarounds, and make informed decisions about internet service.

Related Articles

NAT and IPv4

IPv6 Transition

Network Configuration

Explore More

Key takeaways: - CGNAT adds ISP-level NAT, sharing public IPs among customers - Uses 100.64.0.0/10 address space - Prevents port forwarding and hosting services - Causes gaming, VoIP, and P2P issues - Detect by checking if WAN IP is 100.64.x.x - Solutions: Request public IP, use IPv6, VPN, or cloud services - Temporary measure until IPv6 adoption - ISPs use it to conserve IPv4 addresses - Not inherently bad, but has limitations - IPv6 is the long-term solution

Whether you're troubleshooting connection issues, planning to host services, or simply understanding your internet connection, knowledge of CGNAT empowers you to work within its limitations or find appropriate solutions.

ad placeholder image ad placeholder image
Three funny piglies - an illustration ippigly.com