How to determine a node ( Computer System / User Agent ) uniquely in an external network?

In my website I have implemented a API rate limit ,I am using Ip address of the request as a part of the key and that key will be unique as the IP address is unique, but If 20 system are connected to the same wifi and there are hitting my site at the same time then they all are blocked just after the first request because they all have the same public ip address (IP address of Hub/Router) from which I am creating my key , and this is a problem, can you suggest a better way to identify a system uniquely in this network so I can provide each system with a rate limit of 20 hits in one min instead of 20 hits for the whole network in one min.?