Inside Zone Check: Hundreds of domain availability results in milliseconds

Your users are searching for a domain name inside your platform. They type a query, hit enter, and wait. By the time availability badges populate, the moment feels sluggish — and some percentage of users have already moved on.

That latency is inherent in the architecture of traditional domain availability checks. Every lookup requires a round-trip to an external registry, and those round-trips take hundreds of milliseconds each. Fan out across a dozen TLDs per query and the math gets painful fast.

Zone Check is the name.com API endpoint built to eliminate that bottleneck. It returns availability results for batches of domain names in milliseconds — no registry round-trips, no multi-second waits. Response times vary by location and implementation, but are consistently orders of magnitude faster than real-time registry lookups. It's the same endpoint that powers Vercel's domain search experience and Railway's domain integration, and it's available to every name.com API customer.

The problem: Domain search was never built for modern speed expectations

Standard domain availability checks — whether via EPP, WHOIS, or a registrar API's checkAvailability endpoint — work by querying a registry in real time. The registry looks up the domain, confirms its status, and returns a result. That process is authoritative and accurate. It's also slow.

A single registry lookup takes 200–500 milliseconds. If your search experience checks availability across 20 TLDs for every query, you're looking at multiple seconds of cumulative latency — even with parallelization and connection pooling. For any platform where domain search sits at a critical conversion moment, that latency is a product problem, not just a performance issue.

The workaround most platforms accepted was to either limit TLD coverage (check fewer extensions per query) or tolerate the lag. Some paid for a separate third-party service just to serve faster results — running two domain vendors in parallel because no single registrar API could handle both speed and purchase.

"One of our partners had two things going: a legacy registrar API for purchases, and then a separate service just to serve faster search results," says Pat Ramsey, Principal Engineer at name.com. "They wanted the speed of a dedicated search product with the reliability of a real registrar behind it. Zone Check was how we delivered both."

What is Zone Check?

Zone Check is a dedicated availability endpoint in the name.com API that checks domain names against a locally cached index of every registered domain — without hitting an external registry.

The data comes from zone files: comprehensive lists of every domain registered under a given TLD, published by registries like VeriSign (.com, .net), PIR (.org), CentralNic, Radix, and hundreds of others. name.com downloads and processes these files as registries publish them, roughly every 24 hours.

Instead of storing gigabytes of raw zone file data, Zone Check compresses it into a bloom filter — a probabilistic data structure optimized for fast membership testing. The entire index of registered domains fits in approximately 1 gigabyte of memory. When you query a domain, Zone Check checks the bloom filter and returns a result in milliseconds — far faster than any registry round-trip.

The result is a fast, high-confidence availability signal for any domain name across hundreds of TLDs — fast enough to power search-as-you-type experiences.

Zone Check is an exact-match availability endpoint. You provide specific, fully qualified domain names (acmecorp.com, acmecorp.net) and it tells you whether each one is registered. If you want keyword-based suggestions — where the API generates a list of relevant domain names for a search term — use the Search endpoint instead.

How it works

1. Send a batch of domain names

Zone Check accepts a POST request with an array of fully qualified domain names. You'd typically generate this list by appending your user's search term to every TLD you want to display.

curl -X POST https://api.name.com/core/v1/domains:zoneCheck \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "domainNames": [
      "acmecorp.com",
      "acmecorp.net",
      "acmecorp.io",
      "acmecorp.dev",
      "acmecorp.app"
    ]
  }'

2. Receive availability results instantly

The response returns each domain with an availability signal. No registry was queried, the bloom filter held the answer in memory.

{
  "results": [
    { "domainName": "acmecorp.com", "available": false },
    { "domainName": "acmecorp.net", "available": true },
    { "domainName": "acmecorp.io", "available": true },
    { "domainName": "acmecorp.dev", "available": false },
    { "domainName": "acmecorp.app", "available": true }
  ]
}

3. Confirm with Check Availability when the user commits

When a user selects a domain, follow up with Check Availability — a real-time registry lookup that confirms the domain is registrable right now.

curl --request POST \
  --url https://api.dev.name.com/core/v1/zonecheck \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "domainNames": [
    "<string>"
  ]
}
'

This two-step pattern is the key integration design: Zone Check handles the expensive fan-out across dozens of TLDs without burning registry lookups on every keystroke. Check Availability handles the authoritative confirmation on the one or two domains the user actually wants.

"They do the Zone Check first for the fast result, and then if a TLD supports premium domains, they go out and fetch pricing to determine if it's a premium," Pat explained. "So there's the fast availability layer, and then a pricing check on top of it. The pricing check can be a little slower, but by that point the user already has results on screen."

Comparing endpoints

Full endpoint docs: See the Zone Check API reference for supported TLDs, batch limits, rate limits, error codes, and complete request/response schemas.

Built for accuracy at speed

Bloom filters can produce false positives (reporting a domain as taken when it isn't) but never false negatives (reporting a domain as available when it's actually registered). The false positive rate is tunable: you trade memory for accuracy.

"Ours is tuned way up, basically to the max, so we aim for zero false positives," Pat said. "And we haven't had anyone report one."

Zone Check operates as a negative check. It doesn't confirm that a domain is available — it checks whether the domain appears in the set of all registered domains. If the domain isn't in the bloom filter, it's likely available. If it is in the filter, it's taken. This distinction matters for how you build on top of it: Zone Check is a fast screening layer, not a registration guarantee. The Check Availability endpoint handles confirmation.

Because the underlying zone file data refreshes on a daily cycle, there's a natural question about accuracy between updates. In practice, the overwhelming majority of domain names don't change registration status within that window — making the refresh cadence a non-issue for search UX. And when edge cases do surface, they inform ongoing improvements to the system.

“Any inaccuracies partners have encountered have resulted in changes on our end to close those gaps," Pat said.

Infrastructure most registrars haven't built

Zone Check requires two things most registrar APIs don't invest in: a bloom filter service running in memory, and active zone file access agreements with registries worldwide.

The bloom filter engineering is the visible part — compressing 10 gigabytes of registered domain data into roughly 1 gigabyte of fast, queryable memory. But the operational foundation is just as important. Every major registry has its own process for granting zone file access, and name.com maintains those relationships to keep data flowing from VeriSign, Identity Digital, CentralNic, Radix, PIR, and other registries.

"That was actually the long tail of this project," Pat said. "Other registrars could build their own Zone Check with a bloom filter — they choose not to, or they haven't put the work in, or they don't see the value."

This is part of what makes Zone Check a managed service rather than a DIY exercise. As a trusted registrar with direct registry relationships, name.com has access to complete zone files and premium domain pricing lists that aren't available to the general public. You don't need to negotiate that access, build bloom filter infrastructure, or maintain a data refresh pipeline. You call an endpoint and get results in milliseconds.

The streaming search pattern: how platfomrs use Zone Check

The most natural starting point for Zone Check is the pattern that Vercel pioneered and Railway adopted: a streaming domain search experience where results populate as the user types.

"For a certain segment of users, they don't care about domain recommendations or clever name-spinning," Pat said. "They just want to see everything that's available for their search term. Zone Check excels at that."

This pattern works for any platform where end users search for and register domains — deployment platforms, website builders, AI app builders, creator tools. The integration surface is small (Zone Check + Check Availability + Register), and the UX improvement is immediate.

"For a certain segment of users, they don't care about domain recommendations or clever name-spinning," Pat said. "They just want to see everything that's available for their search term. Zone Check excels at that."

Who built Zone Check

Zone Check was a collaboration across three roles at name.com:

• Pat Ramsey, Principal Engineer — designed and built the Zone Check service, including the bloom filter architecture and edge-case handling across TLDs.
• Eric Berkow, Lead Software Engineer — wired Zone Check into the name.com API as a standard REST endpoint alongside Check Availability, registration, and the rest of the API surface.
• John Rupp, Sr. Registrar Operations Manager — secured zone file access from registries worldwide and manages the operational relationships that keep Zone Check's data current.

Build the fastest domain search your users have ever seen

Zone Check eliminates the trade-off between speed and coverage. Instead of choosing fewer TLDs for faster results, or more TLDs with multi-second waits — you ship both. Instant results across hundreds of TLDs, backed by authoritative confirmation when it matters.

The endpoint is available to every name.com API customer. No special access required. Sign up, grab your API key, and start building.

Integrate Zone Check into your platform →

• Zone Check API reference — endpoint docs, schemas, supported TLDs
• Check Availability API reference — real-time registry confirmation
• API overview and authentication — getting started
• Quickstart guide — from API key to first domain operation in minutes