ISP4x: Restoring IPv4 End-to-End Connectivity via Stateless Address Extension

The exhaustion of IPv4 addresses has forced ISPs to deploy Carrier-Grade NAT (CGNAT), a stateful middlebox that breaks end-to-end connectivity for billions of users. IPv6 could restore addressability but requires an incompatible protocol stack that, after 25 years, remains only partially deployed. We present ISP4x, a single-stack architecture that provides a scalable proof-of-concept for improving end-to-end connectivity within IPv4. ISP4x places a 16-bit extension in the standardized IPv4 Options field by leveraging the IPswen addressing framework to expand the effective address space by a factor of 65, 536 per base IP, and introduces Stateless NATswen, an O(1), protocol-agnostic address translation requiring zero per-flow state. We implement an XDP-accelerated edge gateway and evaluate it in a 500-node emulated ISP network. Under 90% load, ISP4x sustains 86% of link capacity while CGNAT degrades to 46%, with p99 jitter of 1.8 ms versus 45 ms. Notably, ISP4x requires zero per-flow memory, removing the state-exhaustion vulnerability inherent in CGNAT. These preliminary results suggest that ISP4x offers a pragmatic, incrementally deployable path toward restoring peer-to-peer connectivity within and across participating ISP domains without abandoning IPv4 infrastructure.