# Introduction to TCP/IP Reference Model:- The ARPANET, a research network, which was developed by U.S. department of defense, connected with hundreds of universities and government installations using leased lines. When satellite and radio networks were added to these network, compatibility arises. The exiting protocols are not compatible with new networks. So no communication in between old and new networks. A new architecture is needed which connects multiple networks in a seamless way. A flexible architecture was needed since applications with diverse requirements were envisioned, ranging from transferring files to real time speech transmission. This architecture was developed and known as TCP/IP reference model. TCP/IP stands for Transmission Control Protocol / Internet Protocol. The TCP/IP is a set of protocols, or a protocol suite, that defines how all transmission are exchanged across the Internet. The following diagram gives the pictorial representation of the TCP/IP Reference Model. # TCP/IP Model contains following Layers:- 1. Host-to-Network Layer: Host-to-network layer is the first layer of the four layer TCP/IP model. This layer corresponds to the physical and data link layer of the OSI model. Host-to-network layer, defines details of how data is physically sent through the network, including how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire. The protocols included in Host-to-network layer are Ethernet, Token Ring, FDDI, X.25, Frame Relay etc. 2. Internet Layer: Internet layer is the second layer of the four layer TCP/IP mode. Internet layer pack data into data packets known as IP datagrams, which contain source and destination address (logical address or IP address) information that is used to forward the datagrams between hosts and across networks. The internet layer is also responsible for routing of IP address. Internet layer’s job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol). 3. Transport Layer: Transport layer is the third layer of four layer TCP/IP model. The purpose of Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data. It is designed to allow peer entities on the source and destination hosts to carry on a conversation. We call it as end-to-end communication. For this communication, two end-to-end protocols TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are used. 4. Application Layer: Application layer is the top most 4th layer of the TCP/IP model. Application layer defines TCP / IP application protocols and how host programs interface with Transport layer services to use the network. Application layer includes all the higher – level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), SNMP (Simple Network Management Protocol), TFTP (Trivial File Transfer Protocol), DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc. The TCP/IP protocol suite was developed prior to the OSI model. Therefore, the layers in the TCP / IP protocol suite did not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers i.e. host-to-network, internet, transport and application. However, when TCP/IP is compared to OSI, we can say that the host-to-network layer is equivalent to the combination of the physical and data link layers. The internet layer is equivalent to the network layer, and the application layer is roughly doing the job of the session, presentation, and application layers with the transport layer in TCP/IP taking care of part of the duties of the session layer. So in this section, we assume that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport and application. The first four layers provide physical standards, network interfaces, internetworking and transport functions that correspond to the first four layer of the OSI model. The three topmost layers in the OSI model, however, are represented in TCP/IP by a single layer called the application layer as shown in the Figure. TCP/IP is a hierarchical protocol made up of n-interactive modules with specific functionality. These modules are not interdependent. In OSI model, every layer is having predefined functions. The layers in TCP/IP protocol suite contain relatively independent protocols that can be mixed and matched depending on the needs of the system. Every upper layer protocol is supported by one or more lower level protocols. Thank You! Visit Again!!
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