Firefox AS Number Extension


The AS Number Extension displays the AS Number, and consequently the Internet Service Provider, of every website visited along with some additional interesting information for those interested in how the Internet works below the shiny surface.
All data is updated daily and the prefix to AS number mapping is from a real DFZ BGP feed.


Firefox 3.6 ASNumber Extension: Release 1.6 - 20100311

Who are we:

Internet Business Solutions AG
BGP Experts, Traffic & Network Engineering Gurus


The extension transmits the IP address of the server of every website visited. No cookies are set or read from the ASNumber server and no per-user or browser tracking or logging is done. Only aggregated statistics on the popularity of the the looked up AS Numbers are computed. No information, other than those aggregates, is made available to third parties.


Q: When I visit a server via IPv6 is shows "AS n/a". Does it support IPv6?
A: No, we don't support IPv6 lookups yet. So far the focus has been on making IPv4 work because that's where the vast majority of sites is located. IPv6 support is on our TODO list but it requires quite some infrastructure changes and won't happen in the next days probably.

Q: Why is sometimes not all information for an AS Number shown?
A: We take our data from the RIR whois entries for the AS numbers and sometimes the ISP does not properly update or name its entry. Sometimes there is confusion as which RIR is responsible for a particular AS number, particularily those from before the mid-nineties. We are working on better data merging and manually fixing certain popular AS number entries. This is a tedious process however and may take some time.

Q: How up to date is the information?
A: We reload the database and actual BGP data from the DFZ (default-free zone) every 24 hours.

Q: What new features have you planned?
A: We have lots of ideas how to impove the information displayed by the extension. Some are more easy implemented than others. Displaying more information does not require to reinstall or update the extension.

Q: I have this great idea for a new ASNumber feature, who do I tell?
A: We'd like to hear about it and may get around implementing it. Please send us an email with your idea at

Q: Why does Firefox behave differently with the extension installed?
Q: Why does Firefox crash with the extension installed?
A: First make sure that you have switches to doing lookups synchronously (right-click on ASNumber field, Settings, DNS Resolver) and restart Firefox. If the problem persists it is either a bug in Firefox itself or a bug triggered by the extension. Please send us a bug report with as specific information as possible about your operating system, Firefox version, extension version, the website you visited when it crashed and any other information about the circumstances when the problem happend to The more detailed information we have the better we can track down the problem.