Following IPTables rules exist on my system as some preventive measures against DDoS attacks-A INPUT -s 255.0.0.0/8 -j LOG --log-prefix Spoofed source IP -A INPUT -s 255.0.0.0/8 -j DROP-A INPUT -s 0.0.0.0/8 -j LOG --log-prefix Spoofed source IP-A INPUT -s 0.0.0.0/8 -j DROPIs it restricting private IP Address Group with the assumption that packets originating from private IP Address range from Internet are actually spoofed IP Address. Also, How to calculate the corresponding IP Address range from the above subnets ? 
Understanding Iptables rules to prevent DDoS
rhel;iptables;ip;firewall
The -A INPUT -s adds a rule for any packets with the source subnets specified on the line.Therefore, in your example, you are logging and dropping all packets that have a source IP address that starts with 255 and all that start with 0, such as 255.1.2.3.4 or 0.56.78.90The idea here is that there should will never be packets that start with those addresses and therefore if they do appear, they must be spoofed.There are actually many more invalid or reserved IP addresses that the ones you've listed which could be added to that list.There are plenty of resources on subnets and plenty of online subnet calculators to help you out.  In your case, the /8 states that the first 8 bits of the IP address specify the network and all the others (the remaining 24) specify a host within that subnet.  In both examples, the first octet (8 bits) is the network and the other three octets can be any value. Your 255.0.0.0/8 therefore can be from 255.0.0.0 to 255.255.255.255 and your 0.0.0.0/8 can be from 0.0.0.0 to 0.255.255.255.