A denial of Service (DoS) attack works on the following principle:

A.
MS-DOS and PC-DOS operating system utilize a weaknesses that can be compromised and permit them to launch an attack easily.

B.
All CLIENT systems have TCP/IP stack implementation weakness that can be compromised and permit them to lunch an attack easily.

C.
Overloaded buffer systems can easily address error conditions and respond appropriately.

D.
Host systems cannot respond to real traffic, if they have an overwhelming number of incomplete connections (SYN/RCVD State).

E.
A server stops accepting connections from certain networks one those network become flooded.

Explanation:
Denial-of-service (often abbreviated as DoS) is a class of attacks in which an attacker attempts to prevent legitimate users from accessing an Internet service, such as a web site. This can be done by exercising a software bug that causes the software running the service to fail (such as the "Ping of Death" attack against Windows NT systems), sending enough data to consume all available network bandwidth (as in the May, 2001 attacks against Gibson Research), or sending data in such a way as to consume a particular resource needed by the service.