Introducing TCP/IP
TCP/IP is at the very core of all things networking, so I really want to ensure that you have
a comprehensive and functional command of it. I’ll start by giving you the whole TCP/IP
backstory, including its inception, and then move on to describe the important technical goals as defined by its original architects. And of course I’ll include how TCP/IP compares
to the theoretical OSI model.
TCP/IP and the DoD Model:
The DoD model is basically a condensed version of the OSI model that comprises four instead
of seven layers:
uu Process/Application layer
uu Host-to-Host layer/or Transport
uu Internet layer
uu Network Access layer/or Link
Figure 3.1 offers a comparison of the DoD model and the OSI reference model. As you can
see, the two are similar in concept, but each has a different number of layers with different
names. Cisco may at times use different names for the same layer, such as both “Network
Access” and “Link” used to describe the bottom layer.

The DoD and OSI models
Application
OSI Model
Session
Presentation
Transport
Network
Data Link
Physical
Process/
Application
DoD Model
Host-to-Host
Internet
Network
Access
When the different protocols in the IP stack are discussed, the layers of the
OSI and DoD models are interchangeable. In other words, be prepared for
the exam objectives to call the Host-to-Host layer the Transport layer!
A vast array of protocols join forces at the DoD model’s Process/Application layer.
These processes integrate the various activities and duties spanning the focus of the OSI’s
corresponding top three layers (Application, Presentation, and Session). We’ll focus on
a few of the most important applications found in the CCNA objectives. In short, the
Process/Application layer defines protocols for node-to-node application communication
and controls user-interface specifications.
The Host-to-Host layer parallels the functions of the OSI’s Transport layer, defining
protocols for setting up the level of transmission service for applications. It tackles issues
like creating reliable end-to-end communication and ensuring the error-free delivery of
data. It handles packet sequencing and maintains data integrity.
The Internet layer corresponds to the OSI’s Network layer, designating the protocols
relating to the logical transmission of packets over the entire network. It takes care of the
addressing of hosts by giving them an IP (Internet Protocol) address and handles the routing
of packets among multiple networks.
At the bottom of the DoD model, the Network Access layer implements the data exchange
between the host and the network. The equivalent of the Data Link and Physical layers of the
OSI model, the Network Access layer oversees hardware addressing and defines protocols for
the physical transmission of data. The reason TCP/IP became so popular is because there were
no set physical layer specifications, so it could run on any existing or future physical network!
The DoD and OSI models are alike in design and concept and have similar functions in
similar layers. Figure 3.2 shows the TCP/IP protocol suite and how its protocols relate to
the DoD model layers.
In the following sections, we will look at the different protocols in more detail, beginning
with those found at the Process/Application layer.

The TCP/IP protocol suite:
DoD Model
Telnet
TFTP
FTP
SMTP
LPD
NFS
SNMP
X Window
Process/
Application
Host-to-Host
ICMP ARP RARP
IP
Internet
Network
Access
TCP UDP
Ethernet Fast
Ethernet
Gigabit
Ethernet
10 Gig
Ethernet
The Process/Application Layer Protocols
Coming up, I’ll describe the different applications and services typically used in IP networks,
and although there are many more protocols defined here, we’ll focus in on the protocols most
relevant to the CCNA objectives. Here’s a list of the protocols and applications we’ll cover in
this section:
*Telnet
* SSH
*FTP
*TFTP
*SNMP
*HTTP
*HTTPS
*NTP
*DNS
*DHCP/BootP