I just found the following : https://books.google.co.uk/books?id=ZO09CgAAQBAJ&pg=PA96&lpg=PA96&dq=$binary_

To conserve the space occupied by the key we use $binary_remote_addr It evaluates into a binary value of the remote IP address

So it seems I should be doing this instead to keep the key in memory for that IP small to reduce the memory footprint.

limit_req_zone $binary_http_cf_connecting_ip zone=one:10m rate=30r/m;
limit_conn_zone $binary_http_cf_connecting_ip zone=addr:10m;

http://www.networkflare.com/