Zero Subnet

I just took a practice exam and one of the sims I missed was due to the zero subnet. It stated that it wanted me to find the subnet mask with a max of 6 hosts. The IP range is 192.168.1.0 /24. Use the first available host on the first subnet for R1 S0 and the second available host on the first subnet for R2 S0. I came up with the mask /29 witch was correct and the first host address as 192.168.1.1. R2 S0 address was 192.68.1.2. I thought I was correct on those also. They said the correct first available host was .9 and the second was .10. In the explantion I finally understood the zero subnet deal but how do you know when it comes into play when they don't mention it in the question?