[DGD] Firewalls, whitelists, and dynamic IPs

[Shentino]

Lately I've been trying to nail down an IP address range for a remote
server I work with to whitelist for security reasons.

And I seem to be cursed with an ISP that has a rather fragmented IP
space and on top of that is rather fond of reassigning me to another
IP every time I look at my routing information.

I've tried about twice now to nail my pool down but so far I've
uncovered three different netblocks in whois.

Is there an easy way to find out this information other than trial and
error combined with whois lookups?