1. TCP/IP

2. The idea behind TCP/IP is exactly the same we explained about the OSI reference model: when transmitting data, programs talk to the Application layer, which in turn talks to the Transport layer, which then talks to the Internet layer, which then talks to the Network Interface layer, which sends frames over the transmission media (cable, air, etc).

3. TCP/IP

4. There are 4 layers. TCP/IP isn’t the name of a specific protocol, but the name of a protocol stack, i.e., a set of protocols. Each individual protocol used on the TCP/IP stack works on a different layer. For example, TCP is a protocol that works on the Transport layer, while IP is a protocol that works on the Internet layer.

5. TCP/IP It is possible to have more than one protocol on each layer. They won’t conflict with each other because they are used for different tasks. For example, when you send out e-mails, your e-mail program talks to the SMTP protocol located on the Application layer. Then this protocol, after processing the e-mails received from your e-mail program, sends them to the layer below, Transport. There data will be processed by the TCP protocol. When you browse the web, your web browser will also talk to the Application layer, but this time using a different protocol, HTTP, as this is the protocol in charge of processing web browsing.

6. TCP/IP Here is a brief explanation of each TCP/IP layer: Application: Programs talk to this layer. Several different protocols can be used on this layer, depending on the program you are using. The most common are HTTP (for web browsing), SMTP (for sending e-mails), POP3 (for receiving e-mails) and FTP (for transferring files). Transport: Everything about the Transport layer from the OSI reference model is valid for the TCP/IP Transport layer. Two different protocols can be used on this layer, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP is used for transmitting user data (like web browsing and e-mails) Internet: Everything about the Network layer from the OSI reference model is valid for the TCP/IP Internet layer. Several protocols can be used on this layer and the most common one is the IP protocol. Network Interface: This layer is in charge of sending data to the transmission media. What is inside this layer will depend on the kind of network you have. If you are using an Ethernet Network (the most common network type) you will find the three Ethernet layers (LLC, MAC and Physical – LLC stands for Logic Link Control and MAC stands for Media Access Control) inside this TCP/IP layer. The Physical layer from Ethernet networks corresponds to the Physical layer from the OSI model, while the other two layers (LLC and MAC) correspond to the Data Link layer from the OSI model.

7. TCP/IP After processing the program request, the protocol on the Application layer will talk to another protocol from the Transport layer, usually TCP. This layer is in charge of getting data sent by the upper layer, dividing them into packets and sending them to the layer below, Internet. Also, during data reception, this layer is in charge of putting the packets received from the network in order (because they can be received out-of-order) and also checking if the contents of the packets are intact.

8. TCP/IP On the Internet layer we have the IP (Internet Protocol), which gets the packets received from the Transport layer and adds virtual address information, i.e., adds the address of the computer that is sending data and the address of the computer that will receive this data. These virtual addresses are called IP addresses. Then the packet is sent to the lower layer, Network Interface. On this layer packets are called datagrams.

9. TCP/IP The Network Interface will get the packets sent by the Internet layer and send them over the network (or receive them from the network, if the computer is receiving data). What is inside this layer will depend on the type of network your computer is using. Nowadays almost all computers use a type of network called Ethernet (which is available in several different speed grades; wireless networks are also Ethernet networks) and thus you should find inside the Network Interface layer the Ethernet layers, which are Logic Link Control (LLC), Media Access Control (MAC) and Physical, listed from up to bottom. Packets transmitted over the network are called frames.