An ASN (Autonomous System Number) refers to a block of IP addresses.

Most large organizations or ISPs are allocated a number of ASNs.

ASNs can be useful to block an entire organization. Instead of manually having to add hundreds of IP addresses into a firewall rule, you can simply block the entire ASN, which blocks all IPs within that ASN.

There are many search tools online such as https://www.ultratools.com/tools/asnInfo.

For example, searching for Google returns a number of ASNs.

Alternatively:

dig google.com
 
; <<>> DiG 9.16.1-Ubuntu <<>> google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 2531
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
 
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;google.com.			IN	A
 
;; ANSWER SECTION:
google.com.		300	IN	A	216.58.198.206
 
;; Query time: 24 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Mon Jan 04 15:55:10 GMT 2021
;; MSG SIZE  rcvd: 55

Then

whois -h whois.cymru.com " -v 216.58.198.206"
 
AS      | IP               | BGP Prefix          | CC | Registry | Allocated  | AS Name
15169   | 216.58.198.206   | 216.58.198.0/24     | US | arin     | 2012-01-27 | GOOGLE, US

NOTE: ASN is shown here as 15169. —- ===== Determine which IPs actually fall into an ASN ===== Taking AS41264 from the above search as an example: <code bash> whois -h whois.radb.net – '-i origin AS41264' | grep ^route | grep -v route6 | cut -d“ ” -f7 | most </code> returns: <code bash> 74.125.63.0/24 74.125.58.0/24 74.125.60.0/24 104.132.0.0/16 104.132.0.0/23 104.132.2.0/23 104.132.4.0/23 104.132.6.0/23 104.132.8.0/23 104.132.10.0/23 … </code>