1. Network Layer
and
Transport Layer

2. Introduction
The Network layer and Transport layer are responsible for moving messages from end to end in a network. The transport layer performs three functions: establishing end to end connections, addressing and packetizing. The network layer on the other hand performs two functions: addressing and routing. It uses several standard transport and network layer protocols that specify how packets are to be organized and the widely used protocol is the TCP/IP, the protocol used in the Internet.

3. How Do Transport and Network Layers Work?
Transport layer links the application software in the application layer with the network and is responsible for the end to end delivery of the message. It also accepts outgoing messages from the application layer (e.g. Web and ) and packetizes and addresses them for transmission. The network layer takes the messages from the transport layer and routes them through the network by selecting the best path from computer to computer through the network.

4. Transmission Control Protocol / Internet Protocol (TCP/IP)
The Transmission Control Protocol/Internet protocol is the transport/network layer protocol used in the Internet and is widely used today. As the name implies, TCP/IP has two parts:
TCP is the transport layer protocol that links the application layer to the network layer. It performs packetizing (breaking the data into smaller packets), numbering them, ensuring each packet is reliably delivered, and putting them in the proper order at the destination.

5. Transmission Control Protocol / Internet Protocol (TCP/IP)
IP is the network layer protocol and performs addressing and routing. IP software is used at each of the intervening computer through which the message passes; it is IP that routes the message to the final destination.
There are two forms of IP currently in use today. The older form is IP version 4 (IP v4) and IP version 6 (IP v6).

6. Transmission Control Protocol / Internet Protocol (TCP/IP)

7. Network Layer
The Network Layer is located at the 3rd position in the seven layer OSI model. It offers serviceable and practical ways of transmitting variable length data series from a source to a destination  through one or more networks, while preserving the class of service and error handling tasks.

8. Functions of Network Layer
Path Determination / Routing
Logical Addressing
Packet Forwarding
Error Handling and Diagnostics

9. Path Determination / Routing
Moving data across a series of interconnected networks is probably the defining function of the network layer. It is the job of the devices and software routines that function at the network layer to handle incoming packets from various sources, determine their final destination, and then figure out where they need to be sent to get them where they are supposed to go.
Routing is the process of selecting the fastest route or path through the network from the sender to the receiving computer.

10. Logical Addressing
The network layer uses a network layer address. The network layer protocol used on the Internet is IP (Internet Protocol). It translates a specific web address into an IP Address. This process is similar in using a phonebook to go from someone’s name to his or her phone number.
Each and every component participating in a network communication has a logical address. After determining the fastest route available, addressing is done by finding the address of the next computer.

11. Error Handling and Diagnostics
Packet Forwarding
In the process of routing the packets to their final destination, the Network Layer needs to keep forwarding the packets to concerned intermediate nodes.
Error Handling and Diagnostics
The Network Layer also handles various communication errors, routing flaws, and status exchanges.

12. Transport Layer
The transport layer is the fourth layer of the OSI Model and exists between the Session Layer and Network Layer. The transport layer is used to actually transfer data from one application to another and can be used to stream data from one device to another. The most popular transport layer, the Transmission Control Protocol, is the foundation for Internet access all over the world.

13. Functions of Transport Layer
Establishing end to end connection including linking the application layer to the network
Addressing
Packetizing

14. Linking to the Application Layer
Most computers have many application layer software packages running at the same time. Users often have Web browsers, programs, word processors and so on is used at the same time on client computers. Likewise, many servers act as web servers, mail servers and FTP servers. When the transport layer receives an incoming message, it decides to which application program it should be delivered. Meaning, a webpage request must be send to a web server and the like.

15. Linking to the Application Layer
With TCP/IP, each application layer software package has a unique port address. Any message sent to computer must tell TCP the application layer port address that is to receive the message.
On the Internet, all port addresses for popular such as the Web, and FTP (File Transfer Protocol) and so on have been standardized. Web server has a port address of 80. FTP servers use port 21 and so on.

16. Addressing
Addressing means to find the address of the ultimate destination computer. After establishing the connection, transport layer determines the address of the computer (either a client or a server) where the message should be delivered. Addressing is done to ensure that the data is transmitted correctly and for the network layer to determine the fastest route available.

17. Packetizing
Packetizing means to take one outgoing message from the application layer and break it into a set of smaller packets for transmission through the network. It also means to take the incoming set of smaller packets from the network layer and reassemble them into one message for the application layer which is also called fragmentation and reassembly.

18. Definition of Terms
File Transfer Protocol (FTP) – enables users to send and receive files over the Internet. There are two types of FTP sites: closed(which require users to have an account and a password) and anonymous which permit anyone to use them.
Transmission Control Protocol (TCP/IP) – is probably the oldest networking standard, developed for ARPANET, and now used in the Internet. One of the most commonly used network protocols.