Help me find my possible dynamic IP range.

February 10, 2011 2:29 PM

How do I determine the range of possible addresses that my ISP might assign me?

I have a web application that I'm trying to whitelist by IP address, but one of the customers has dynamic IP from his ISP. For the short term, while I develop a dyndns.org solution with a script on my end, I would like to just allow his possible range. Is there a tool I can use to do that? I've seen results on old forums with things like adsl-68-127-1[01]-[n].dsl.pltn13.pacbell.net, where [01] could be either zero or 1, and [n] is good for 0-255...

posted by joecacti to computers & internet (5 answers total)

Perhaps you could call the ISP and ask?
posted by adamrice at 2:38 PM on February 10, 2011

You can ask the particular ISP. Or Google the info.
For instance, here are the dynamic ranges used by Comcast.
posted by Thorzdad at 2:40 PM on February 10, 2011

1. Have you asked them?
2. http://bgp.he.net/ will tell you (with *decent* accuracy) your allocated prefix. Warning: for a telco, this might be *large* like a /13 or something. If you type your ip in the search box you might get a result like:

68.13.1.225 (ip68-13-1-225.om.om.cox.net)

Announced By
Origin AS Announcement Description
AS22773 68.0.0.0/12 Cox Communications Inc.
AS22773 68.13.0.0/17 Cox Communications Inc.

3. memail me with your actual IP. I do this stuff for a living, and can give you a tighter answer.
posted by gregglind at 2:40 PM on February 10, 2011

You might try poking around ARIN.
posted by Hylas at 2:41 PM on February 10, 2011

For a quick and dirty solution you can usually just use a /24. For a better estimate do a whois on the ip with the links already provided. You'll probably get a /15 or so which is pretty huge. It's not clear whether you want to allow anyone from that ISP or just anyone "close" in allocation (whatever that means.) From your question you seem to be wanting to use reverse DNS but that is not a reliable method as a) many ISPs don't have rDNS for all addresses and b) if a person has been delegated control of their reverse DNS they can make it say anything, so it can't be trusted for authentication.
posted by Rhomboid at 3:10 PM on February 10, 2011