Over the years, the Internet has evolved from a tool for the research community to an indispensable network connecting over a billion nodes world wide. There are many security threats existing on the Internet, one of them is the denial-of-service attack (DoS). In this thesis, we study effect of denial-of-service attacks arising from TCP SYN flooding. SYN flooding attack has been widely observed world-wide, and occupies about 90% of the DoS attacks. We examine the effects of the attacks on individual host, and the underlying network infrastructure carrying the SYN flood packets. In laboratory, we deploy isolated network set-ups, to test the effects of the attacks on both the network and host. Finally, we design a queuing upper bound model to estimate the probability of connection loss on a host under a SYN flood attack. We compare the results from our upper bound model with results from selected models in the literature.