TCP/IP model

TCP/IP model

TCP/IP model. Approach to the theory of network design.

Hello again, I will continue to crush my niece with abstract concepts, in the previous post I talked about the theoretical OSI reference model and today we continue with another model that has practical implementation and does not stay in the theoretical framework of reference.


The TCP/IP (Transmission Control Protocol/Internet Protocol) model is the result of the development of a switched communications network within the United States Department of Defense that offered point-to-point communications. The set of TCP/IP protocols offered the addressing specifications, and how the information should be packaged to guarantee minimum communications services, and so that any manufacturer could design their products with these common bases. This protocol began as a military project, although it was later opened to the general public. At the same time, ISO published the OSI model for communications networks, so it was also defined a TCP/IP model. The TCP/IP model is a four-tier hierarchy model, each layer is built on its predecessor and its services and functions are variable with each type of network.

  • The network access layer includes all physical conversions for sending information through a channel and identifying local computers using Mac addresses.
  • The Internet layer offers connectivity to any computer that uses that protocol. Their form of identification is IP addresses.
  • The transport layer is responsible for establishing the equipment-to-equipment connection channels.
  • The application layer defines those protocols with a specific objective that makes use of the services of the lower layers.

TCP/IP Model – Layer 1 – Network Access

The first layer of the TCP/IP model incorporates all the appropriate signal conversion and coding systems to work according to the physical medium used, similar to the physical layer of the OSI model. Within the network access layer are also included all the definitions and protocols necessary for communication within the network. To identify the computers on the network, MAC (“Media Access Control”) addresses are used. These addresses are composed of a 48-bit length identifier that is divided into two blocks, the first 24 identify the card manufacturer and the other 24 identify the card itself. The MAC address is built into any computer that has network access and is a priori unique (although it is not exactly so and I will tell you that later).

In summary, the network access layer of the TCP/IP model covers the same needs as the first two layers of the OSI model, although some protocols that overlap with the third layer of the OSI model are included in the TCP/IP model. For example, the ARP (Address Resolution Protocol) and RARP (Reverse Address Resolution Protocol) protocols allow the conversion of IP addresses (OSI network layer), with MAC addresses, (OSI data link layer).

TCP/IP Model – Layer 2 – Internet

The Internet layer allows us to have global connectivity and access resources that are outside our immediate environment. The name of the Internet comes from the ability of this layer to communicate through different and distant networks. In this layer, as in the network layer of the OSI model, we find methods, protocols and specifications to send information from one “host” to another through intermediate points if it is the case. This layer also incorporates error detection systems and knowledge of the network status. Unlike the OSI model, no link maintenance mechanisms between nodes are established in the Internet layer, TCP/IP leaves this functionality for the lower layer of network access. The best known protocol used in layer 3 is the “Internet Protocol” or IP, There is also the IPsec protocol that allows the sending of encrypted information. Currently working with version 4 of the IP protocol, although version 6 has already been standardized.


This protocol identifies the connected equipment uniquely by means of IP addresses. This address is in turn divided into a network identification, which corresponds to the first bits of the address and the identifier of the equipment within that network, which corresponds to the last bits of the address. The IP protocol manages the fragmentation of the information sent by the different communication channels in such a way that it divides the information into the blocks with the greatest possible capacity to be sent by a specific channel. Through a specific field, it allows the detection of failures in the transfer of information, and ensures that the information is correct before delivering it to the upper layers. It should be noted that the IP protocol does not ensure the reception of the information sent.


It is the evolution of the IPv4 system whose limit on the number of admissible network addresses is beginning to restrict the growth of the Internet and its use. The new standard improves service and provides for the worldwide connection of trillions of equipment continuously and simultaneously. About this, I will write a special post.

TCP/IP Model – Layer 3 – Transportation

The transport layer of the TCP/IP model is equivalent to layer 4 of the same name in the OSI model whose basic objective is to transfer error-free information to the set of elements of the upper layers. Here we find the protocols with the following services:

  • Connection-oriented communications, the best known protocol is TCP (“Transmission Control Protocol”).
  • Offline communications, the best known protocol is UDP (“User Datagram Protocol”).

Communications reliability is achieved using different strategies, forwarding information or simply discarding invalid information. Delivery is guaranteed in the same shipping order thanks to the package identifiers sent. The control of the flow of information tries to avoid punctual saturation and congestion in the network. The port concept allows the sending and management of multiple communications between “hosts”, so that a multiplexing of the communications channel is achieved. In this way we are not limited to a single flow of traffic between two specific teams. Although the best known protocols in this layer are TCP and UDP, there are others such as Stream Control Transmission Protocol (SCTP), IL Transport Protocol, or Structured Stream Transport (SST).

TCP/IP Model – Layer 4 – Application

The application layer is one that uses the connectivity resources offered by the transport layer. Unlike the OSI model, which establishes two more layers, the TCP/IP model delegates these functions to the application protocols themselves, that is, each protocol decides how to manage its sessions and the type of coding and presentation of the data. This is because the TCP/IP model is based on existing protocols (DARPANET) and on this structure specific application protocols were developed to meet new needs.

As in the application layer of the OSI model, the “application term” is used to define the protocol used by the TCP/IP stack, so it should not be confused with the applications used by the end user.

Some examples of application layer protocols can be:

  • FTP (File Transfer Protocol), which incorporates its own presentation system to send information in binary format or in ASCII format.
  • SSH (Secure Shell), which uses the security systems offered by the lower layers.
  • RSTP (Real Time Streaming Protocol), which uses protocols without connection establishment for sending video over the network.

At present, the TCP/IP model is the most used, it has a very high degree of reliability, it is suitable for medium and large networks as well as business. It’s used to connect to the Internet and web servers. And speaking of servers, in the following posts I’ll talk about the physical equipment to begin to better understand the objects used to manage the layers and about the network architecture. I think it is convenient to finish this article right now.

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