I’m afraid we don’t have the answers for your host’s question, @gnfb.
The IP we use at one point in time for one specific factor is not something we can control, or know, or change. That will depend on the location of each user, for example, among other things.
A good way to figure something out about this is to have your host look at the logs, and search for all requests coming from the Jetpack by WordPress.com
user agent.
Ultimately, though, they will need to whitelist all our IPs. Every site owners have tons of ways of interacting with our services, in different interfaces. We don’t always come from the same one IP ??
If your host has trouble whitelisting so many IPs and wants to know more about that, they’re welcome to send us an email about it. We will put them in touch with one of our network wranglers who can explain them why we need to do this.
You and your host can also have a look at the blog of our very own Barry: there are a lots of interesting insights, videos and links about our infrastructure ??