What is the New Subnet Mask and the IP Address Range of the first Network?

500 hosts would require minimum of 9 bits, as 2^9 = 512. Keeping the lat bit of the third byte reserved for hosts, and the rest seven for subnets, we have first seven bit as 1, making the third byte to be 254. Thus Subnet mask is 255.255.254.0. Now the first subnet will be having subnet id as 0, ie. all the seven bits as 0 in the third byte. And for the first address, we will have first seven bits as zero and last bit as 1 in the 4th Byte (Note: All zeros is not a valid IP, and neither is all 1s). So the first address should ideally be 150.20.0.1. Accordingly the last address would be 150.20.1.254