What is an ASN (autonomous system number)?

An ASN (autonomous system number) is the ID number of a network on the internet. Every internet provider, mobile network, cloud and large company that runs its own routing has one: Telkom is AS37457, Vodacom is AS29975, Google is AS15169 and Cloudflare is AS13335. Your ASN is shown on the FernTool homepage next to your ISP, and the IP lookup shows it for any address.

What an “autonomous system” is, in plain words

The internet is a network of networks, more than 70 000 of them. Each one (an autonomous system) controls its own block of IP addresses and tells its neighbours “send traffic for these addresses to me”. They do this with a routing protocol called BGP, and the ASN is the name tag on those announcements. When you load a website, your traffic hops from your ISP’s AS to one or more others until it reaches the site’s AS.

How to find your ASN

Open the FernTool homepage. Under Connection details you’ll see ISP and ASN, for example “AS37611”.
For someone else’s IP, paste it into the IP lookup.
To see who registered an ASN and its contact details, type it (e.g. AS37457) into the WHOIS lookup.

South African ISP and mobile network ASNs

Network	Main ASN(s)
Telkom	AS37457, AS37251
Vodacom	AS29975, AS36994
MTN	AS16637, AS12091
Cell C	AS37168
Rain	AS37105, AS33762
Afrihost	AS37611
Cool Ideas	AS37680

More networks, and whether each uses CGNAT, are on the South African ISP pages. If you buy fibre from an ISP like Afrihost, websites see the ISP’s ASN, not the fibre company’s (Vumatel, Openserve).

Why your ASN matters

Location: IP location databases work per network, which is why your IP may show Johannesburg when you’re in Durban.
Gaming and speed: how well your ISP’s AS connects to game and streaming networks (peering) affects ping more than your package speed.
VPN detection: if an IP’s ASN belongs to a hosting company, it’s probably a VPN or server. See how to check if an IP is a VPN.
Blocking and abuse: network admins block or report whole ASNs that send attacks or spam.

Who needs their own ASN?

Home users and most businesses never need one; you use your ISP’s. An organisation gets its own ASN when it connects to two or more providers with its own IP addresses (multihoming), for example a hosting company or bank. In Africa they’re issued by AFRINIC, usually through a member ISP. Numbers 64512–65534 and 4200000000–4294967294 are private ASNs, used inside networks and never seen on the public internet.

ASN vs IP address vs ISP

IP address: The address of one connection, e.g. 41.13.0.1 (what is an IP address?).
ASN: The network that owns and routes a whole block of those addresses.
ISP: The company you pay. Big ISPs have one or several ASNs; small resellers use their supplier’s.

Frequently asked questions

What is my ASN?

Open the FernTool homepage: the ASN line under Connection details shows the network your IP belongs to, such as AS37457 for Telkom.

What does AS mean before a number?

AS stands for autonomous system. AS13335 simply means autonomous system number 13335, which belongs to Cloudflare.

Can an ASN identify a person?

No. An ASN identifies a whole network, usually an ISP with thousands or millions of customers. It tells you the provider, not the person.

How many ASNs are there?

More than 70 000 ASNs are active on the public internet. ASNs used to be 16-bit (up to 65 535) and are now 32-bit, allowing over 4 billion.