How Mobile Carriers (CGNAT) Allocate Dynamic Regional Subnets
A deep dive into mobile telecommunication data architectures, exploring how cellular towers tunnel traffic to centralized core gateways and share public IPv4 pools.
Quick Answer
Mobile carriers assign dynamic subnets using large-scale Carrier-Grade NAT (CGNAT) and centralized 3GPP packet cores. Because mobile networks support billions of handsets, devices receive internal RFC 6598 addresses (100.64.0.0/10) that are tunneled via GPRS Tunneling Protocol (GTP) to centralized regional Packet Gateways (PGW/UPF), sharing a pool of rotating public egress IPs.
- RFC 6598 CGNAT: Mobile handsets share outbound public IPv4 exit pools with thousands of subscribers.
- Centralized backhaul: Cellular traffic exits at regional hub data centers, not at the physical radio tower.
- 464XLAT transition: Modern 5G carriers deploy IPv6-only cores with dual-stack translation for legacy IPv4 apps.
1. The 3GPP Mobile Core Architecture
Unlike fixed broadband where an optical line terminal (OLT) connects directly to local ISP edge routers, mobile cellular data involves an encapsulated tunnel hierarchy:
2. Access Technology and Subnet Allocation Comparison
Compare how IP addressing and NAT behave across mobile cellular, fixed broadband, and satellite networks:
| Network Type | IPv4 Addressing Model | IPv6 Support | IP Stability | Inbound Port Forwarding? |
|---|---|---|---|---|
| Mobile 4G LTE | CGNAT (RFC 6598) Shared Pool | Dual-Stack or 464XLAT | Highly dynamic (Changes on reconnect) | Blocked by carrier firewall |
| Mobile 5G Standalone | IPv6-first with 464XLAT / NAT64 | Native globally unique /64 | Dynamic (Assigned per session) | IPv6 possible; IPv4 blocked |
| Fixed Fiber / Cable | Public dynamic or CGNAT (/10) | Native /56 or /64 prefix delegation | Semi-static (Retained for weeks) | Supported on public IPv4 |
| Satellite (Starlink) | CGNAT (100.64.0.0/10) by default | Native /56 prefix delegation | Dynamic per ground station | Supported via IPv6 or Priority IPv4 |
| Corporate Mobile APN | Dedicated Private APN or Static IP | Dual-stack or private IPv4 | Completely static | Fully supported within private APN |
Mobile Cellular Subnet Allocation Decision Tree
[ Smartphone / Cellular Device Connects ]
|
+-------------+-------------+
| |
[ Native IPv6 ] [ Legacy IPv4 ]
Carrier issues Carrier issues
Globally unique RFC 6598 CGNAT
/64 prefix via SLAAC 100.64.x.x internal IP
| |
| 464XLAT / NAT64 Gateway
| translates to shared public IPv4
| |
+-------------+-------------+
|
[ Tunneled via GTP to Core PGW / UPF ]
Centralized regional data center POP
Egresses to Internet with shared Carrier ASN
3. Why Cellular IP Geolocation Jumps Erratically
Users frequently notice that browsing from a mobile phone in a small town indicates their IP is in a city hundreds of miles away:
- Consolidated Packet Gateways: Carriers do not install Internet egress routers at every cell tower. Instead, thousands of towers across multiple states backhaul traffic to a single centralized PGW datacenter (e.g., Dallas, Chicago, or Atlanta).
- BGP Anycast Routing: Mobile carriers announce large
/16and/18subnets from multiple regional egress points simultaneously. - Dynamic Pool Rotation: As thousands of smartphones connect and disconnect, CGNAT port allocation engines constantly recycle IP assignments across the subscriber base.
4. Practical Diagnostics: Checking Mobile IP Parameters
Commands and techniques to diagnose mobile hotspot or cellular IP parameters:
Frequently Asked Questions
Why does my cellular IP address change every time I turn Airplane Mode on and off?
Toggling Airplane Mode disconnects your cellular modem from the base station and tears down your GTP (GPRS Tunneling Protocol) session. When the connection re-establishes, the mobile Packet Gateway (PGW/UPF) dynamically assigns an available address from its current regional dynamic pool.
Are all mobile phones behind Carrier-Grade NAT (CGNAT)?
For IPv4, virtually 100% of global cellular connections utilize CGNAT (RFC 6598: 100.64.0.0/10). Mobile operators lack sufficient public IPv4 addresses to assign a unique public IPv4 to billions of smartphones. However, modern 4G and 5G networks assign a native, globally unique public IPv6 /64 prefix to each device.
Can I host a home server or forward ports over a 4G/5G mobile hotspot?
Standard mobile cellular connections do not allow inbound port forwarding due to symmetric CGNAT firewalls at the carrier core. To host inbound services over 5G, you must either use an IPv6 address (if the carrier does not firewall incoming IPv6 traffic), purchase a specialized business static IP APN, or use an overlay reverse proxy / VPN tunnel.
Why does my 5G connection show an IPv6 address but websites say I have an IPv4 address?
Most modern mobile networks operate IPv6-mostly or IPv6-only architectures using 464XLAT (RFC 6877). Your smartphone has an internal CLAT translator that converts legacy IPv4 app traffic into IPv6 packets, which are transported across the carrier core to a carrier PLAT (NAT64 gateway) that assigns a shared public IPv4 address for exit.
Data & Technical Sources
- IETF RFC 6598 — IANA-Reserved IPv4 Prefix for Shared Address Space
- IETF RFC 6877 — 464XLAT: Dual-Stack Mobile IPv4/IPv6 Transition
- 3GPP TS 23.501 — System Architecture for the 5G System (5GS)