Autonomous System (AS)
A collection of IP networks under a single administrative domain with a unified routing policy. Each AS has a unique ASN (Autonomous System Number). CDNs, ISPs, and large enterprises each operate their own AS.
Full Explanation
An Autonomous System (AS) is a network or collection of networks that all fall under a single organization's administrative control and share one unified routing policy. Think of it as a "network domain" on the internet. Every AS gets a unique number called an ASN (Autonomous System Number), assigned by IANA through regional registries like ARIN, RIPE, and APNIC. These numbers are what routers use to figure out where to send traffic across the global internet.
CDNs, ISPs, cloud providers, and large enterprises all operate their own AS numbers. Some well-known examples: Cloudflare is AS13335, Fastly is AS54113, Google is AS15169, and Akamai operates multiple ASNs across different regions. When you traceroute a request, each hop between AS boundaries is where BGP does its work, deciding which path your packets take.
BGP (Border Gateway Protocol) is the protocol that ASes use to exchange routing information with each other. Each AS announces the IP prefixes it owns and can reach, and neighboring ASes use that information to build routing tables. This is how the entire internet figures out how to get packets from point A to point B across multiple networks.
You can look up the AS for any IP address or domain using whois or online tools like bgp.he.net. This is useful for debugging routing issues or understanding which network is serving your traffic.
# Look up the ASN for an IP address
whois -h whois.radb.net 104.16.132.229
# Or use the dig command to query Team Cymru's service
dig +short 229.132.16.104.origin.asn.cymru.com TXT
# Returns: "13335 | 104.16.0.0/13 | US | arin | 2014-03-28"
# Look up details about an ASN
whois -h whois.radb.net AS13335Examples
To find out which AS is serving your CDN traffic:
# Resolve the domain and look up the ASN
dig +short cdn.example.com
# 104.16.132.229
curl -s https://stat.ripe.net/data/prefix-overview/data.json?resource=104.16.132.229 | python3 -m json.tool
# Shows AS13335 (Cloudflare)Knowing the ASN tells you which CDN or network is actually handling the request, which is critical for debugging multi-CDN setups or verifying failover behavior.
Video Explanation
Frequently Asked Questions
A collection of IP networks under a single administrative domain with a unified routing policy. Each AS has a unique ASN (Autonomous System Number). CDNs, ISPs, and large enterprises each operate their own AS.
To find out which AS is serving your CDN traffic:
# Resolve the domain and look up the ASN
dig +short cdn.example.com
# 104.16.132.229
curl -s https://stat.ripe.net/data/prefix-overview/data.json?resource=104.16.132.229 | python3 -m json.tool
# Shows AS13335 (Cloudflare)Knowing the ASN tells you which CDN or network is actually handling the request, which is critical for debugging multi-CDN setups or verifying failover behavior.