Why a Website is Down in Your City but Live in London: DNS Propagation & Anycast

1. The Myth of Instant DNS Updates

When a webmaster changes the IP address of their domain name, that change does not instantaneously propagate to every computer on Earth.

Every DNS record comes with a TTL (Time-To-Live) value measured in seconds. If a domain had a TTL of 86400 (24 hours), recursive DNS servers operated by Comcast, Vodafone, or AT&T will continue serving the cached old IP for up to 24 hours after the authoritative server changed.

During this transition window, visitors in one country will reach the new server, while visitors in another city will continue hitting the old (often offline) server.

2. Anycast BGP Routing and Regional Outages

Modern tech companies use BGP Anycast routing: hundreds of servers around the world advertise the exact same IP address. Routers across the internet direct your traffic to the topologically closest server.

However, if the Anycast point-of-presence (PoP) in Chicago crashes or suffers fiber cuts, traffic in the Midwest may fail to load the site, even while European and Asian traffic routes normally to London and Tokyo datacenters.

3. Verifying Authoritative Nameservers with Command-Line Tools

If you suspect your local ISP is serving stale DNS records, you can bypass your ISP resolver and query the domain's authoritative nameservers directly using command-line diagnostic utilities:

# Query Cloudflare public DNS directly
nslookup example.com 1.1.1.1

# Trace the full authoritative DNS delegation chain
dig +trace example.com