Introduction to the DoD / TCP/IP Model

by Jason Zandri - June 25, 2010

The TCP/IP model, sometimes referred to as the Department of Defense (DoD) Model, is a four-Layer model that was developed in the early 1970s for the Defense Advanced Research Projects Agency (DARPA) internetwork project that eventually grew into the modern day Internet.

This article will be a brief overview of the model itself and the tie in to the Cisco Certified Network Associate Routing & Switching exam (640-802 CCNA), as well as the Network+ exam.

Tie in to the CCNA and Network+ exams

Both the 640-802 CCNA exam and the Network+ exam test for some of the common knowledge of the DOD / TCP/IP Model.

For the CCNA 640-802 exam this information is tested as part of the "Describe how a network works" domain as part of the following subtopics:

• Use the OSI and TCP/IP models and their associated protocols to explain how data flows in a network
• Describe the purpose and basic operation of the protocols in the OSI and TCP models
• Select the components required to meet a network specification

Beyond this domain and the subtopics you'll need a good, general understanding of the model across some of the other domains as well but this is the primary area of focus.

For the Network+ exam most of your need of this material will be a part of domain 4.0 - Network Management. Having said that, there are secondary needs for knowing the information peppered throughout the exam in other domains. A couple of examples are calling upon the knowledge to know and understand where in the model certain protocols function as well as knowing at what layers hardware devices such as routers, switches, bridges, etcetera work.

Breakdown of the Four Layers of the TCP/IP Model

In summary the four layers of the TCP/IP model are broken as follows:

The Network Interface Layer is responsible for delivering data over the wire at the physical level. There are a number of different protocols that function at this layer depending on the type of physical network that is in use. This layer of the TCP/IP Model maps to the data-link and physical layers of the OSI model.

The Internet Layer is designed to provide connectionless delivery service for communications. It also allows for the breaking down of the data into individual packets / frames on the transmitting side and their reassembly on the receiving side. The Internet Layer also provides routing functions as needed for network nodes to to interoperate with other networks. This layer of the TCP/IP Model maps to the network layer of the OSI model

The Host-to-Host Layer (sometimes also referred to as the or Transport or the Transmission Layer) is responsible for the building up and tearing down of the communication sessions between connected network nodes. This layer is also provides error detection and correction. This layer of the TCP/IP Model maps to the transport layer of the OSI model.

The Application Layer is designed to leverage the application protocols and further defines how the host programs interface with transport layer services to use the network. This layer of the TCP/IP Model maps to the three top layers of the OSI Model - Session at layer 5, Presentation at layer 6 and Application at layer 7.

Protocol Use at each of the TCP/IP Model Layers

At each layer of the TCP/IP Model there are associated protocols that are in use. These too loosely map to the same ones that are found as part of the OSI Model.

At the Application layer you can find many but some of the more common ones include:

• BOOTP - Bootstrap Protocol
• DHCP - Dynamic Host Configuration Protocol
• DNS - Domain Name System (Service) Protocol
• FTP - File Transfer Protocol
• HTTP - HyperText Transfer Protocol
• IMAP - IMAP4, Internet Message Access Protocol (version 4)
• IRCP - Internet Relay Chat Protocol
• LDAP - Lightweight Directory Access Protocol
• LPD - Line Printer Daemon Protocol
• MIME (S-MIME) - Multipurpose Internet Mail Extensions and Secure MIME
• NFS - Network File System
• NIS - Network Information Service
• NNTP - Network News Transfer Protocol
• NTP - Network Time Protocol
• POP - POP3, Post Office Protocol (version 3)
• RDP - Remote Desktop Protocol
• RPC - Remote Procedure Call
• SMB - Server Message Block
• SMTP - Simple Mail Transfer Protocol
• SNMP - Simple Network Management Protocol
• SNTP - Simple Network Time Protocol
• SSH - Secure Shell
• TELNET - Terminal Emulation Protocol of TCP/IP
• TFTP - Trivial File Transfer Protocol
• X.400 - Message Handling Service Protocol
• X.500 - Directory Access Protocol (DAP)

At the Host to Host / Transport layer you can find these common protocols:

• ATP - AppleTalk Transaction Protocol
• FCP - Fiber Channel Protocol
• NBF - NetBIOS Frames protocol
• SPX - Sequenced Packet Exchange
• TCP - Transmission Control Protocol
• UDP - User Datagram Protocol
• At the Internet Layer you can find:
• IPv4 - Internet Protocol v4
• IPv6 - Internet Protocol v6
• ICMP - Internet Control Message Protocol
• IGMP - Internet Group Management Protocol
• IPSec - Internet Protocol Security

At the final layer of the TCP/IP Model, the Network Interface Layer, you can find:

• Ethernet
• Token Ring
• FDDI - Fiber Distributed Data Interface
• X.25
• Frame Relay
• RS-232
• v.35

These are not fully comprehensive lists but are examples of the more common protocols that are functioning at these different levels of the TCP/IP Model

In this article we reviewed the tie in of the TCP/IP / DOD Model to the CCNA and Network+ exams as well as took a look at the breakdown of the Four Layers of the TCP/IP Model

We wrapped up with a quick look at some of the protocols that are in use at each of the TCP/IP Model Layers

Thanks for investing your time in my Introduction to the DOD / TCP/IP Model article.

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