Denial of service attacks do profit from fundamental weaknesses of TCP/IP protocols, as well as from incorrect implementations of TCP/IP protocol stacks. Attacks, which profit from fundamental weaknesses are e.g. SYN Flood and Smurf. Attacks aiming at incorrect implementations are all attacks, which operate with incorrectly fragmented packets (e.g. Teardrop), or which work with falsified sender addresses (e. g. Land). In the following some of these attacks are described, their effects and possible countermeasures.