So, I took Cisco Classes (networking 1-4) at my Community College back in 2013. I did very well and aced each of the 4 classes... But, of course, life through a curve ball and here I am and it's almost 2018. I picked up all the recent Cisco books, ICND1 CCENT and ICND2 CCNA Study guides and began reading ICND1 CCENT. One thing I noticed, the DoD model. Back in 2013, the DoD model wasn't even talked about. My teacher, however, compared and contrasted the TCP/IP model with the OSI model. Now, in all the new books I'm reading, the author, Todd Lammle, will confusingly skip from the OSI to the DoD to the TCP/IP model when he tries to illustrate a point. Why?