Denial of Service(DoS) attack:
Its main target is a Web server. Due to this attack the Web server/computer forced to restart and this results refusal of service to the genuine users.
If we want to access a website first you have to type the web site address in the URL and press Enter key, the browser requests that page from the web server. Dos attacks send huge number of requests to the web server until it collapses due to the load and stops functioning.
