How many layers are defined within the US Department of Defense (DoD) TCP/IP Model?

The TCP/IP protocol model is similar to the OSI model but it defines only four layers:

Application

Host-to-host

Internet

Network access

Reference(s) used for this question:

http://www.novell.com/documentation/nw65/ntwk_ipv4_nw/data/hozdx4oj.html

and

KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and Network Security (page 84).

also see:

http://en.wikipedia.org/wiki/Internet_Protocol_Suite#Layer_names_and_number_of_layers_in_the_literature