Anycast vs Unicast DNS: Why It Matters for Performance

When a user in Tokyo and a user in London both query the same domain, where does their request actually go? With unicast DNS, both queries travel to the same physical server, wherever it happens to be, even if that means crossing an ocean. With anycast DNS, each query is automatically routed to the nearest server out of many that share the same address. That difference shapes how fast your domain resolves, how well it survives traffic spikes, and how resilient it is against attacks.

Anycast is one of the main reasons major DNS providers can offer low latency worldwide and absorb massive denial-of-service attacks. Understanding how it differs from unicast explains a lot about why managed DNS performs the way it does, and why it matters for your domain.

This guide explains both routing methods, how anycast works, the advantages it provides, and where each fits.

Unicast: One Address, One Destination

Unicast is the standard, traditional model of network addressing. Each IP address corresponds to exactly one network interface on one server in one location. When traffic is sent to a unicast address, it travels to that specific server, wherever in the world it physically sits.

For DNS, this means a unicast nameserver has an IP address that maps to a single machine. A query from anywhere in the world is routed across the internet to that one machine. If your nameserver is in a data center in Virginia, a user in Australia querying it sends their request all the way to Virginia and waits for the response to travel all the way back.

Unicast is simple and predictable. You know exactly which server handles a request. But it has inherent limitations for global services: latency depends on distance, capacity depends on that single server (or a manually load-balanced set), and resilience depends on that one location staying online.

Anycast: One Address, Many Destinations

Anycast breaks the one-address-one-server assumption. With anycast, the same IP address is announced from multiple locations simultaneously. Many physically separate servers, potentially hundreds, all share the identical IP address.

When a user sends a query to that anycast address, the internet's routing system (BGP, the Border Gateway Protocol) directs the query to the topologically nearest instance, meaning the one with the shortest network path. A user in Tokyo reaches the Tokyo instance. A user in London reaches the London instance. Both used the exact same IP address, but the network delivered each to a different physical server based on proximity.

This is the key insight: with anycast, the network itself handles the routing to the closest server, transparently, without the user or the DNS query doing anything special. The same destination address yields different physical destinations depending on where the request originates.

How Anycast Actually Works

Anycast relies on BGP, the protocol that routers use to exchange information about how to reach different parts of the internet. Here's the mechanism:

1. Multiple servers announce the same IP prefix. An operator places servers in many locations (points of presence, or PoPs) around the world. Each location announces, via BGP, that it can reach the anycast IP address.
2. Routers learn multiple paths to the same address. Internet routers receive these announcements and see that the anycast address is reachable through several different paths.
3. Each router picks the best (shortest) path. Using BGP's path-selection logic, each router forwards traffic for the anycast address along the shortest path it knows, which leads to the nearest announcing server.
4. Queries naturally distribute to the closest PoP. The result is that queries from any given region are delivered to the nearest server, with no central coordination required. The routing fabric of the internet does the work.

If one PoP goes offline, it stops announcing the route, and routers automatically redirect traffic to the next-nearest PoP. This failover is automatic and fast, which is a major resilience benefit.

Why Anycast Matters for DNS

Lower Latency Worldwide

DNS resolution is latency-sensitive. Every web page load involves multiple DNS lookups, and each one adds to the time before content appears. With unicast, users far from your nameserver experience high DNS latency. With anycast, queries reach a nearby PoP, dramatically reducing resolution time for a global audience. A query that might take 200ms round-trip to a distant unicast server could take 10-20ms to a nearby anycast PoP.

DDoS Resilience

This is one of anycast's most important benefits. In a DNS-based DDoS attack, attack traffic floods your nameservers. With unicast, all that traffic converges on a single server, which is quickly overwhelmed. With anycast, attack traffic is distributed across all the PoPs based on where it originates. A botnet in Asia hits the Asian PoPs; a botnet in Europe hits the European PoPs. The attack is naturally spread across the entire anycast network rather than concentrated on one target, dramatically increasing the capacity available to absorb it.

Automatic Failover

If a PoP fails or is taken offline for maintenance, BGP automatically reroutes queries to the next-nearest PoP. There's no manual intervention, no DNS change, no waiting for TTLs to expire. The failover happens at the routing layer in seconds. This provides a level of resilience that's difficult to achieve with unicast.

Load Distribution

Query load is naturally spread across all PoPs by geography. No single server handles the entire world's queries. This distribution means each individual server handles a manageable portion of traffic, improving performance and reducing the risk of any one server being overwhelmed by legitimate load.

Anycast and the DNS Root Servers

The clearest real-world demonstration of anycast is the DNS root server system. There are 13 root server identities (named A through M), but there aren't just 13 physical machines. Through anycast, those 13 addresses are served by over 1,000 physical server instances distributed around the globe.

This is how the root of the entire DNS hierarchy stays fast and resilient despite handling enormous query volumes and being a constant target for attacks. When you query a root server, you reach the nearest instance via anycast, which is why root server queries are fast no matter where you are. The root system's use of anycast is a big part of why DNS as a whole is so robust.

Comparing the Two

When Each Makes Sense

Anycast Is the Right Choice When

• You serve a global audience and want low DNS latency everywhere
• DDoS resilience matters (and for most public-facing domains, it does)
• You want automatic failover without manual intervention
• You're using a managed DNS provider (most major ones use anycast by default)

Unicast Can Be Sufficient When

• Your audience is concentrated in one region near your server
• You're running a small or internal DNS setup where global performance isn't a concern
• You don't have the infrastructure or expertise to operate an anycast network (which requires multiple PoPs, BGP, and the ability to announce IP prefixes)

For most organizations, the practical path to anycast is simply using a managed DNS provider. Operating your own anycast network requires significant infrastructure: multiple points of presence, BGP peering arrangements, and provider-independent IP space. This is why anycast is a major advantage of managed DNS, as we discussed in our self-hosted vs managed DNS guide. Managed providers have already built global anycast networks, and you benefit from them automatically just by using the service.

Anycast Doesn't Replace Monitoring

Anycast improves performance and resilience, but it doesn't guarantee your DNS is correct or secure. An anycast network will faithfully serve whatever records it's configured with, including misconfigured or maliciously altered ones, to users worldwide, quickly. The speed and reach of anycast mean that a bad record propagates to your entire global audience just as efficiently as a good one.

This is where monitoring remains essential regardless of your routing model. DNS Assistant monitors what your nameservers actually return to queries, catching:

• Unauthorized record changes that anycast would otherwise serve globally without question
• DNSSEC validation issues across your domains
• NS delegation problems including lame delegation
• Resolution failures or unexpected answers, with real-time alerting via email, Slack, Microsoft Teams, webhooks, and SMS

Whether your DNS is served via unicast or a global anycast network, what matters for security is whether the right records are being served, and that requires continuous monitoring.

Check Your DNS

Use the DNS lookup tool at dnsassistant.com/tools to query your domain's records and nameservers. Run a Free Domain Risk Report for a comprehensive view of your DNS configuration.

For continuous monitoring of your DNS regardless of how it's routed, with real-time alerting, sign up at dnsassistant.com.