This was something I recently had looked into after a colleague had asked about it.
The eaiest way IMHO is to use the vCloud API Query Service (http://blogs.vmware.com/vsphere/2012/04/quickly-finding-objects-using-the-vcloud-api-query-service.html)
Here's a screenshot of an External Network called "Ext-Network-1" for a VCD 5.1 deployment
Step 1 - Perform a GET on the following URL: https://[VCD-IP]/api/query?type=externalNetwork
Step 2 - This should return a list of NetworkRecords which will be your available external networks, here's a screenshot using FF RestClient
Step 3 - Once you have identified the "name" property of your External Network, you will need to do a GET on it's HREF
Step 4 - From here, you can clearly see the current IP Range as well as the allocated IP Addresses
If you want more details on the allocated IP Addresses, you'll see in the "Link" there is /allocatedAddresses that you can perform a GET on that URL which will give you further details
Hopefully this helps you get what you're looking