Unexplainable usage of Internet I am a domestic internet user on my Mac through a cable modem with a wifi router provided by a very large cable TV &amp; broadband provider. I have a 7 Mbps connectivity with a download limit of 25GB.  I have been using this for the last 18 months and my usage used to be below 12 GB per month.  The last two months I have been getting huge bills from the ISP claiming that my usage has gone above 40 to 50 GB. There is no change in the usage pattern or even the software deployed in the last many months but the billed usage currently is unexplainable. The router does not have wifi range out side my home where one Mac and two iPhones are only used. The internet usage on the iPhone is also limited to some emails and simple browsing which we have been doing for 18 months.

Now, sometimes the usage statistics from the tool provided by the ISP shows 700 to 800 MB usage in 20 to 30 minutes even when all my systems are idle.

How is it possible for me to plug this hole?
The easiest (and cheapest) way is to check the data usage through Activity Monitor. Just type in the name in Spotlight to launch it, and navigate to the Network tab. You can sort by bytes sent/received, though I think you'll have to keep it running to get historical data by app (otherwise shows only total data sent/received). 

For a complete solution, where you would get prompts for all apps trying to access the internet, and the ability to create filters etc., you can check out Little Snitch 3 (https://www.obdev.at/products/littlesnitch/index.html).