DoS or denial-of-service attacks can be considered as a strategy designed to keep the valid traffic from accessing the target of the attack. Distributed denial-of-service (DDoS) attacks happen when the attack comes from the multiple concurrent sources. The one of the differences between DoS and DDoS is that the latter is harder to prevent using the firewall. But the main difference between these two is that in the DoS attack, the target will be attacked from the single source, whereas in the DDoS attack the target will be attacked from the multiple sources.