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Is there a way to determine the range of addresses a dynamic IP home user will communicate from when connected to their ISP?

The known factors are:

  • User's public IP address
  • The ISP's Name
  • PTR domain for the IP

What I'm really looking for is:

  • A subnet block to use in a very specific firewall rule
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1 Answer

up vote 3 down vote accepted

Do a whois against the RIR who assigned the IP addresses. Among the output will be something like:

$ whois -a 69.18.136.8
. . . . . .
NetRange:       69.18.128.0 - 69.18.223.255
CIDR:           69.18.192.0/19, 69.18.128.0/18
. . . . . .

Their IP will be somewhere in the ISP's allocated range. If you want to narrow it down more you'll have to speak to the ISP and see if they'll tell you how their dynamic addresses are allocated. They probably won't.

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In the example above I picked a DNS server at the ISP I used to work for, which gives one of the netblocks they use. The dynamic IP range for our DSL customers is substantially smaller (a few /24s), but we reserve the right to move those ranges around as needed. – voretaq7♦ Jun 11 '12 at 18:48
Yeah, the whois for this address gives me a /10 net block. Perhaps a bit too big for what I'm looking for. – Hyppy Jun 11 '12 at 18:54
@Hyppy Usually there's a subdelegation. You might have to traverse a few whois servers to get a narrow answer. – Jeff Ferland Jun 11 '12 at 20:03
@JeffFerland That doesn't tell the whole story though - the ISP had discontiguous ranges, so if you asked our rwhois server about the subdelegation you might miss a bunch of subnets that were in the dynamic address pool. – voretaq7♦ Jun 11 '12 at 20:06

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