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ASN (Autonomous System Number)

Also calledautonomous system numberas number
Definition

An ASN (autonomous system number) is the ID of one independently run network on the internet, such as an ISP, a cloud provider or a big company, used to route traffic between networks.

Explanation

In depth

The internet is tens of thousands of separately run networks that agree, through the routing protocol BGP, how to pass traffic to each other. Each one is an autonomous system with a unique number. Cloudflare is AS13335, Google AS15169, Meta AS32934, Fastly AS54113. ASNs matter to ordinary users for three reasons. First, censorship is applied per network, so results differ by ASN even inside one country. In OONI's tests of vimeo.com in Iran from 15 July to 15 October 2026, 447 of 568 tests on Mobinnet (AS50810) were confirmed blocks, against 210 of 2,198 on TCI (AS58224), where most results were anomalies instead. Second, one routing mistake can affect the world: in February 2008 Pakistan Telecom (AS17557) announced a route for YouTube's addresses that leaked and briefly took YouTube offline globally. Third, sites judge visitors by the network their address belongs to. Home broadband ASNs look like people; cloud and hosting ASNs look like servers, so they get more CAPTCHAs and firewall blocks. AnyProxy's website, anyproxy.site, sits on Cloudflare's AS13335. Its proxy regions run on a cloud provider's network, so sites see a datacenter ASN, one reason some sites block proxy traffic outright.

In practice

ASN (Autonomous System Number) in the real world

OONI's per-network counts for vimeo.com in Iran (15 Jul - 15 Oct 2026) show why the ASN matters. MCI AS197207: 587 confirmed blocks of 811 tests. Mobinnet AS50810: 447 of 568. Respina AS42337: 347 of 388. Irancell AS44244: 374 of 920, plus 465 anomalies. TCI AS58224: 210 of 2,198, with 1,886 anomalies. Same site, same country, same period, and a different failure pattern on each network. That is why one person's 'works for me' does not transfer to another ISP.

Sources: OONI Explorer: vimeo.com tests in Iran, 15 Jul - 15 Oct 2026 · RIPE NCC — YouTube hijacking case study (2008)

Measured · 2026-10-05

Which network carries each popular site

From a DomainScan scan of 37 popular sites.

Own network · 7Dailymotion, Reuters, Telegram, Yandex, Wikipedia, Claude, AliExpress
Google · 6YouTube, Snapchat, Google, Gemini, Spotify, Disney+
Cloudflare · 6Vimeo, X, Discord, Signal, ChatGPT, Perplexity
Fastly · 6Twitch, Reddit, Pinterest, BBC, CNN, The Guardian
Meta · 4Instagram, Facebook, WhatsApp Web, Messenger
Amazon (AWS) · 4Al Jazeera, SoundCloud, Netflix, Amazon
Akamai · 2TikTok, eBay
Microsoft · 2Bing, DuckDuckGo

Questions about ASN (Autonomous System Number)

How do I find a website's ASN?
Look up its IP address in a BGP or IP-lookup tool such as bgp.he.net. Our site pages show each site's network from a DomainScan scan.
Why do Instagram and WhatsApp break at the same time?
Both run on Meta's own network, AS32934. A block or outage on that network affects all Meta apps together.
Why do some sites block proxies and VPNs?
Their addresses belong to datacenter and cloud ASNs. Sites can block a whole network number in one rule, which is what a 'Sorry, you have been blocked' page often means.

Related terms

IP addressAn IP address is the number that identifies a device or server on a network, so packets know where to go; every website you visit sees one for you.ISP (Internet Service Provider)An ISP (internet service provider) is the company that connects you to the internet, gives you a public IP address, routes your traffic and usually carries out national blocking orders.IP blockingIP blocking stops traffic to particular server addresses, so a site stays unreachable even when its name is looked up correctly.Datacenter proxyA data-centre proxy sends your traffic out from an IP address owned by a cloud or hosting company: fast and cheap, but easy for sites to recognise.

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