Introduction to TCP/IP Reference Model | CCNA R&S | TechnoGB

The TCP/IP Reference Model

The Transmission Control Protocol/Internet Protocol (TCP/IP) is the network model used in the current Internet Architecture. The TCP/IP is developed by the Department of Defence’s Project Research Agency (DoD) as a part of the research project of network interconnection to connect the remote devices/machines.

The DoD Model or TCP/IP Reference Model is basically a condensed version of the OSI Model that comprises four instead of the seven layers which are :

  1. Process/Application Layer
  2. Host-to-Host Layer/Transport Layer
  3. Internet Layer
  4. Network Access Layer/Link Layer

In the below figure I have described the comparison between the DoD Model or the TCP/IP Model and OSI Model. Each has a different number of the layer with different names with them, as both the models are similar in concept.

The TCP/IP Reference Model

The Process/Application Layer defines the protocol for node-node application communication and controls user interface specifications. The process/Application Layer of the TCP/IP Reference Model parallel’s the function of OSI’s Application Layer, Session Layer, Presentation Layer.

The Host-to-Host Layer Parallel’s the function of OSI’s Transport Layer. It defines the 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 of TCP/IP Reference Model corresponds to the OSI’s Network Layer. It takes care of the addressing of hosts by giving them a unique IP (Internet Protocol) address and handles the routing of packets among the multiple networks.

At the bottom of the DoD Model or the TCP/IP Reference Model, the Network Access Layer implants the data exchange between the host and the network. This Layer is equivalent to the Datalink Layer and Physical Layer of the OSI Model. The Network Access Layer oversees the hardware addressing and defines the 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.

Now Let us take a look at different protocols of TCP/IP reference Model or DoD Model in more detail.

Introduction to TCP/IP reference Model

Before moving to the above protocols of different layers of TCP/IP reference Model let us first know about the what does a Protocol basically means in the language of Networking. So, Protocols are the set of rules which govern every possible communication over a network. The Protocols describes the movement of data between the source and destination or the Internet.

Protocols of Process/Application Layer :
    • Telnet: Telnet allows a user on a remote client machine, called the Telnet Client, to access the resources of another machine. The drawback of Telnet is that there are no encryption techniques available within the Telnet Protocols, so everything must be sent in the clear text.


    • SSH: Secure Shell (SSH) Protocol sets up a secure session that’s similar to Telnet over a standard TCP/IP connection and is employed for doing things like logging into systems, running programs on remote systems and moving file from one system to another system. And it does all this while maintaining an encrypted connection.


    • FTP: File Transfer Protocol (FTP) actually lets us transfer files, and it can accomplish this between any two machines using it. FTP allows access to both directories and files and can accomplish certain types of directory operations, such as relocating into different ones. FTP functions are limited to listing and manipulating directories, typing file contents, and copying files between hosts.


    • TFTP: Trivial File Transfer Protocol (TFTP) is the stripped-down, stock version of FTP. TFTP is fast and so easy to use. TFTP doesn’t offer the abundance of functions that FTP does because it has no directory-browsing abilities, meaning that it can only send and receive files.

    • SNMP: Simple Network Management Protocol (SNMP) collects and manipulates valuable network information. It gathers the data by polling the device on the network from a Network Management Station (NMS) at fixed or random intervals.
    •  HTTP: Hypertext Transfer Protocol (HTTP) is used to manage communications between web browsers and web servers and opens the right resources when you click a link, wherever that resource may actually reside.
    • HTTPS: Hypertext Transfer Protocol Secure (HTTPS) is also known as a secure Hypertext Transfer Protocol. It uses the Secure Socket Layer (SSL). It is the secure version of the HTTP that arms us with a whole bunch of security tools for keeping transaction between web browsers and server secure. It is what our browser needs to fill out forms, sign in, authenticate, and encrypt an HTTP message when we do things like making an online reservation, accessing online banking, or buying something online.
    • NTP: Network Time Protocol (NTP) is used to synchronize the clocks on our computer to one standard time source. This protocol works by synchronizing devices to ensure that all the computers on a given network agree on the time.
    • DNS: Domain Name Service (DNS) resolves hostnames, internet names such as (Fully Qualified Domain Name). A Fully Qualified Domain Name (FQDN)  is a hierarchy that can logically locate a system based on its domain identifier. An IP address identifies hosts on a network and the Internet as well, but DNS was designed to make our lives easier. Think about this: What would happen if you wanted to move your web page to a different service provider? The IP address would change and no one would know what the new one was. DNS allows you to use a domain name to specify an IP address. You can change the IP address as often as you want and no one will know the difference.


    • DHCP: Dynamic Host Configuration Protocol (DHCP) assigns IP Address to hosts. It allows for easier administration and works well in small to very large network environments. Many types of hardware can be used as a DHCP Server, including a Cisco Router. There is a lot of information a DHCP server can provide to a host when the host is requesting an IP address from DHCP Server, here is the list of some common types of information a DHCP server can provide:
      • IP Address
      • Subnet Mask
      • Domain Name
      • Default Gateways
      • DNS Server Address
      • WINS Server Address

Protocols of Host-Host Layer/Transport Layer :
  • TCP: Transmission Control Protocol (TCP) takes a large block of information from an application and breaks them into segments. It numbers and sequences each segment so that the destination’s TCP stack can put the segments back into the order the application intended. After these segments are sent on the transmitting host, TCP waits for an acknowledgment of the receiving end’s TCP virtual circuit session, retransmitting any segments that aren’t acknowledged.


  • UDP: User Datagram Protocol (UDP) is basically the scaled-down economy model of TCP, which is why UDP is sometimes referred to as a thin protocol. UDP does not sequence the segments and does not care about the order in which the segments arrive at the destination. UDP just sends the segments off and forgets about them.


Protocols of Internet Layer :
    • IP: Internet Protocol (IP) looks at each packet’s address. Then using a Routing table where the packet is to be sent next, choosing the best path to send it upon. The protocols of the Network Access layer at the bottom of the DoD model don’t possess IP’s enlightened scope of the entire network; they deal only with physical links (local networks).


    • ICMP: Internet Control Message Protocol (ICMP) works at the network layer and is used by the IP for many different services. ICMP is basically a management protocol and messaging service provider for IP. Its messages are carried as IP datagrams.ICMP packets have the following characteristics:
      • They can provide hosts with information about network problems
      • They are encapsulated within IP datagrams
    • ARP: Address Resolution Protocol (ARP) finds the hardware address of a host from a known IP address. As IP’s detective, ARP interrogates the local network by sending out a broadcast asking the machine with the specified IP address to reply with its hardware address. So basically, ARP translates the software (IP) address into a hardware address.

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