---
title: "Anycast: One IP, Many Locations"
description: "Anycast announces a single IP address from multiple geographic locations so routers automatically direct users to the nearest server, improving latency and resilience without client changes."
---

# Anycast: One IP, Many Locations

**Type:** [What is it?](https://yomu.fyi/visual-explainers/category/what-is-it.md)

![Diagram showing one IP address announced from three geographic locations; user requests route to nearest location; BGP routing directs traffic based on proximity.](https://yomufyi.s3.us-east-1.amazonaws.com/visual-explainers/dd603a9c-f101-4779-a524-f3275ea04f6e/image-cdb44444-3ee5-4bee-8bfd-fd97f9f13b68.webp)

> Anycast announces a single IP address from multiple geographic locations so routers automatically direct users to the nearest server, improving latency and resilience without client changes.

## Explanation

Anycast is a network routing technique where the same IP address is advertised from multiple data centers or edge locations. When a user requests that IP, the routing system (typically BGP) directs their traffic to the geographically closest or best-performing announcement point. Unlike unicast (one IP, one location) or multicast (one sender, many receivers), anycast requires address space, an ASN, BGP announcements, and careful planning around failure scenarios. Engineers use anycast to reduce latency, distribute load, and provide automatic failover without changing application code or client configuration.

**Tags:** [Anycast](https://yomu.fyi/topic/anycast.md), [Architecture](https://yomu.fyi/topic/architecture.md), [BGP](https://yomu.fyi/topic/bgp.md), [DNS](https://yomu.fyi/topic/dns.md), [High Availability](https://yomu.fyi/topic/high-availability.md)

## Related reading

### [ZGateway: Learnings from Putting a Proxy in Front of ZippyDB](https://yomu.fyi/post/zgateway-learnings-from-putting-a-proxy-in-front-of-zippydb.md)
- Company: [Meta](https://yomu.fyi/company/meta.md)
- Author: By Rittik Banik, Yunhao Cao
- Published: Sep 3, 2026
- Type: Explainer

ZGateway is a stateless proxy tier introduced to unify traffic from ZippyDB’s large, diverse client fleet and address the reliability limits of direct access. By terminating TLS, authorizing requests, applying admission control and shaping, and routing through regional tiers, it replaces a many-to-many connection mesh with bounded client-to-gateway and gateway-to-server hops. Its embedded thick C++ client batches and coalesces shard requests, while a shared pipeline supports pure-proxy and read-through-cache tiers; per-use-case metrics, traces, and quota usage provide operational visibility. The gateway handles more than 1 billion operations per second, carries about 40% of ZippyDB traffic, and adds about 6% computational overhead to an average use case, with traffic projected to exceed 60%; production work also consolidated transaction state handling across 100% of transaction traffic without a reliability regression.


### [NVIDIA PAIR Virtual Inference Router Expands Available Compute on Your Local Network](https://yomu.fyi/post/nvidia-pair-virtual-inference-router-expands-available-compute-on-your.md)
- Company: [NVIDIA](https://yomu.fyi/company/nvidia-developer-blog.md)
- Author: Tanya Lenz
- Published: Sep 3, 2026
- Type: Explainer

NVIDIA Personal AI Router (PAIR) addresses the GPU bottleneck created when multi-agent workflows and concurrent local sessions generate many independent inference requests. It acts as a virtual inference router rather than an inference engine, proxying compatible Ollama and LM Studio interfaces, discovering and securely pairing local systems, and scheduling each request onto one eligible node based on readiness, engine state, model availability, workload, and GPU utilization. The agent keeps one familiar connection, while PAIR streams responses back and exposes placement through Jobs and metrics views; it does not merge GPUs, pool VRAM, or split a single request across machines. In an unofficial Hermes five-subagent demonstration, a three-device cluster averaged 8 minutes 48 seconds versus 18 minutes on one RTX Spark laptop, with results explicitly dependent on configuration and workload.


### [parking\_lot: ffffffffffffffff...](https://yomu.fyi/post/parking-lot-ffffffffffffffff.md)
- Company: [Fly.io](https://yomu.fyi/company/fly-io.md)
- Published: May 28, 2025
- Type: Incident / postmortem

Fly.io’s fly-proxy, a Rust-based Anycast router, suffered lockups while managing rapidly changing routing state across thousands of servers. An earlier outage came from an if let expression that kept a read lock through both branches, while later regionalization and lazy loading increased write contention in the Catalog. The investigation traced the new failures to parking\_lot’s try\_write\_for wakeup path: after a waiting writer timed out, it attempted to clear WRITER and WRITER\_PARKED together even though one bit had already been cleared, and wrapping arithmetic produced the corrupted lock value 0xFFFFFFFFFFFFFFFE. The parking\_lot team fixed the defect, after which the lockups did not recur; the team also audited Catalog locking, removed if let patterns and RAII lock guards, and added lock-timing metrics, labeled slow-write logs, and holder tracking.


### [Anycast the easy way](https://yomu.fyi/post/anycast-the-easy-way.md)
- Company: [Fly.io](https://yomu.fyi/company/fly-io.md)
- Published: Mar 23, 2021
- Type: Explainer

Anycast lets an operator announce one IP block from multiple locations so routers connect users to a “nearest” location, but a basic setup requires address space and BGP announcements. The described path involves registering with a Regional Internet Registry, acquiring IPv4 through a broker or obtaining an IPv6 block, and checking purchased addresses for transfer eligibility and abuse-list history. It specifies minimum announcement sizes of an IPv4 /24 or IPv6 /48, notes a roughly $25-per-IPv4-address market rate, and estimates cloud holdings using published ranges. An ASN, available free from RIPE according to the text, groups prefixes and enables independent announcements, while BGP details, traffic direction, and failure scenarios are identified as harder areas left for later.