Use https://whatever.whatever/api/admin/extension/orgNetworks/query to build a list of org networks. For each AdminOrgNetworkRecord returned this gives you the 'OrgName' and the 'linkNetworkName' (external network).
For each AdminOrgNetworkRecord returned, issue a GET against the 'href' (of the form https://whatever.whatever/api/network/04f98e5b-90bb-493c-9e0d-7e9e154f0f7e). In the returned XML, look for
AllowedExternalIpAddresses->IpAddress
Does that give you what you want?
Andy