• 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 layers 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. 

PROTOCOL

• TCP (Transmission Control Protocol)
• UDP (User Datagram Protocol)
• virtual terminal (TELNET)
• file transfer(FTP), and electronic mail (SMTP)
• Domain Name System (DNS),
• HTTP (HyperText Transfer Protocol)
• Stream Control Transmission Protocol (SCTP)
• Address Resolution Protocol (ARP)
• Reverse Address Resolution Protocol (RARP)
• Internet Group Message Protocol (IGMP)
• ICMP (Internet Control Message Protocol)

• TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality; however, the modules are not necessarily interdependent. 
• Whereas the OSI model specifies which functions belong to each of its layers, the layers of the TCP/IP protocol suite contain relatively independent protocols that can be mixed and matched depending on the needs of the system. 
• The term hierarchical means that each upper-level protocol is supported by one or more lower-level protocols. 
• At the transport layer, TCP/IP defines three protocols: Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). 
• At the network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP); there are also some other protocols that support data movement in this layer. 

Physical and Data Link Layers:At the physical and data link layers, TCP/IP does not define any specific protocol. It supports all the standard and proprietary protocols. A network in a TCP/IP internetwork can be a local-area network or a wide-area network.

Network Layer: At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the Internetworking Protocol. IP, in turn, uses four supporting protocols: ARP, RARP, ICMP, and IGMP. 

1. Internetworking Protocol (IP): The Internetworking Protocol (IP) is the transmission mechanism used by the TCP/IP protocols. It is an unreliable and connectionless protocol–a best-effort delivery service. The term best effort means that IP provides no error checking or tracking. IP assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees.
2. IP transports data in packets called datagrams, each of which is transported separately. Datagrams can travel along different routes and can arrive out of sequence or be duplicated. IP does not keep track of the routes and has no facility for reordering datagrams once they arrive at their destination. 
3. Address Resolution Protocol (ARP): The ARP  is used to associate a logical address with a physical address. On a typical physical network, such as a LAN, each device on a link is identified by a physical or station address, usually imprinted on the network interface card (NIC). ARP is used to find the physical address of the node when its Internet address is known.  
4. Reverse Address Resolution Protocol (RARP): Its allows a host to discover its Internet address when it knows only its physical address. It is used when a computer is connected to a network for the first time or when a diskless computer is booted.  
5. Internet Control Message Protocol: The ICMP is a mechanism used by hosts and gateways to send notification of datagram problems back to the sender. ICMP sends query and error reporting messages.  
6. Internet Group Message Protocol: The IGMP is used to facilitate the simultaneous transmission of a message to a group of recipients.  

Transport Layer: Traditionally the transport layer was represented in TCP/IP by two protocols: TCP and UDP.

• IP is a host-to-host protocol, meaning that it can deliver a packet from one physical device to another. 
• UDP and TCP are transport level protocols responsible for delivery of a message from a process (running program) to another process. 
• A new transport layer protocol, SCTP, has been devised to meet the needs of some newer applications. 
• User Datagram Protocol: The User Datagram Protocol (UDP) is the simpler of the two standard TCP/IP transport protocols. It is a process-to-process protocol that adds only port addresses, checksum error control, and length information to the data from the upper layer. 
• Transmission Control Protocol: The TCP provides full transport-layer services to applications. TCP is a reliable stream transport protocol. The term stream, in this context, means connection-oriented: A connection must be established between both ends of a transmission before either can transmit data. At the sending end of each transmission, TCP divides a stream of data into smaller units called segments. Each segment includes a sequence number for reordering after receipt, together with an acknowledgment number for the segments received. Segments are carried across the internet inside of IP datagram. At the receiving end, TCP collects each datagram as it comes in and reorders the transmission based on sequence numbers. 
• Stream Control Transmission Protocol: The SCTP provides support for newer applications such as voice over the Internet. It is a transport layer protocol that combines the best features of UDP and TCP.

Application Layer: The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model. Many protocols are defined at this layer.