Hyper Text Transfer Protocol (HTTP) is a protocol used for transferring web pages (like the one you're reading right now). A protocol is really nothing but a standard way of doing things. If you were to meet the President of the United States, or the king of a country, there would be specific procedures that you'd have to follow. You couldn't just walk up and say "hey dude". There would be a specific way to walk, to talk, a standard greeting, and a standard way to end the conversation. Protocols in the TCP/IP stack serve the same purpose.
The TCP/IP stack has four layers: Application, Transport, Internet, and Network. At each layer there are different protocols that are used to standardize the flow of information, and each one is a computer program (running on your computer) that's used to format the information into a packet as it's moving down the TCP/IP stack. A packet is a combination of the Application Layer data, the Transport Layer header (TCP or UDP), and the IP layer header (the Network Layer takes the packet and turns it into a frame).
The Application Layer
...consists of all applications that use the network to transfer data. It does not care about how the data gets between two points and it knows very little about the status of the network. Applications pass data to the next layer in the TCP/IP stack and then continue to perform other functions until a reply is received. The Application Layer uses host names (like www.dalantech.com) for addressing. Examples of application layer protocols: Hyper Text Transfer Protocol (HTTP -web browsing), Simple Mail Transfer Protocol (SMTP -electronic mail), Domain Name Services (DNS -resolving a host name to an IP address), to name just a few.
The main purpose of the Application Layer is to provide a common command language and syntax between applications that are running on different operating systems -kind of like an interpreter. The data that is sent by an application that uses the network is formatted to conform to one of several set standards. The receiving computer can understand the data that is being sent even if it is running a different operating system than the sender due to the standards that all network applications conform to.
The Transport Layer
...is responsible for assigning source and destination port numbers to applications. Port numbers are used by the Transport Layer for addressing and they range from 1 to 65,535. Port numbers from 0 to 1023 are called "well known ports". The numbers below 256 are reserved for public (standard) services that run at the Application Layer. Here are a few: 25 for SMTP, 53 for DNS (udp for domain resolution and tcp for zone transfers) , and 80 for HTTP. The port numbers from 256 to 1023 are assigned by the IANA to companies for the applications that they sell.
Port numbers from 1024 to 65,535 are used for client side applications -the web browser you are using to read this page, for example. Windows will only assign port numbers up to 5000 -more than enough port numbers for a Windows based PC. Each application has a unique port number assigned to it by the transport layer so that as data is received by the Transport Layer it knows which application to give the data to. An example is when you have more than one browser window running. Each window is a separate instance of the program that you use to surf the web, and each one has a different port number assigned to it so you can go to www.dalantech.com in one browser window and this site does not load into another browser window. Applications like FireFox that use tabbed windows simply have a unique port number assigned to each tab
The Internet Layer
...is the "glue" that holds networking together. It permits the sending, receiving, and routing of data.
The Network Layer
...consists of your Network Interface Card (NIC) and the cable connected to it. It is the physical medium that is used to transmit and receive data. The Network Layer uses Media Access Control (MAC) addresses, discussed earlier, for addressing. The MAC address is fixed at the time an interface was manufactured and cannot be changed. There are a few exceptions, like DSL routers that allow you to clone the MAC address of the NIC in your PC.
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