https://github.com/packethost/packngo/blob/f12198a6cc4b2ec175302b6e22f32f6605defc33/ip.go#L386

The intention is for the cidr request filtering to be submitted as a query parameter. The request may be rejected as an invalid/suspicious request as coded today.

Succinctly, r should be translated to query params and the request body should be nil as it is everywhere else:

packngo $ git grep -n DoRequest.*GET.*| grep -v nil
ip.go:386:	resp, err := i.client.DoRequest("GET", apiPathQuery, r, ar)