Fastnat

Fastnat is a term used in computer networking to describe a class of high-performance NAT (Network Address Translation) implementations designed to handle large numbers of concurrent connections with low latency. The term is not a formal standard and is used by vendors and researchers to refer to optimized NAT fabrics, software, or hardware configurations intended to improve translation throughput and CPU efficiency.

Typical fastnat approaches combine hardware acceleration with software optimizations. Options include kernel or user-space NAT engines that utilize multi-core parallelism, lock-free data structures, preallocated NAT tables, and hardware offload to network interface cards (NICs) or network processing units (NPUs). Some implementations leverage advanced technologies such as eBPF, DPDK, or specialized ASICs to move translation work out of the main CPU.

Functional scope generally includes SNAT and DNAT, port address translation (PAT), and sometimes NAT64 for IPv6–IPv4 interworking. They aim to minimize per-packet processing time, reduce memory churn, and scale to thousands or millions of concurrent connections. Stateful NAT requires connection tracking, while stateless variants can be used for simpler translation patterns with faster throughput but less flexibility.

Deployment contexts include data centers, edge gateways, enterprise firewalls, and service provider routers where high connection density is required. Trade-offs include complexity, hardware cost, debugging difficulty, and potential compatibility with existing firewalls and intrusion prevention systems.

See also: Network address translation, NAT traversal, NAT64, eBPF, DPDK, NIC offload, firewall, edge computing.