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====== Networking - ASN (Autonomous System Number) - About ASN (Autonomous System Number) ======

An **[[https://en.wikipedia.org/wiki/Autonomous_system_(Internet)|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.

----

===== How to find a specific 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.

{{:networking:asn_-_search_-_google.png?800|}}

----

Alternatively:

<code bash>
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
</code>

...then...

<code bash>
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
</code>

<WRAP info>
**NOTE:**  ASN is shown here as 15169, which is another of the above returned ASN in the search.
</WRAP>


----

===== 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>

<WRAP info>
**NOTE:**  The IP addresses within an ASN may change often, so if you do use this for firewalling then update the list regularly.

Also note that the other ASN 15169 determined above also contained the same IP as ASN 41264.  So blocking either of these ASN will still block the IP.
</WRAP>


----

===== References =====

https://www.apnic.net/get-ip/faqs/asn/

https://en.wikipedia.org/wiki/Autonomous_system_(Internet)

https://www.ultratools.com/tools/asnInfo

https://www.nirsoft.net/whois_servers_list.html

https://team-cymru.com/community-services/ip-asn-mapping/