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

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

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

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

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

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