Let’s talk a little bit about how the internet works. To begin, we’ll need to know a little bit about telecommunication. Telecommunication is simply an exchange of information through the use of technology. Technology is very broad and could mean anything - smoke signals, semaphore flags, telegraphs, cellphones and computers all qualify. For each of these methods of communication, there is a system of rules that allow the entities exchanging information to understand one another. This is what we call a protocol.

When computers can exchange data, that is “talk to each other”, they are said to be in a network. The Internet is a network of networks that uses a suite of protocols (aptly) called the Internet protocol suite to be able to communicate with one another.  The many protocols in the Internet protocol suite are separated into four broad layers of functionality.

  1. The Application Layer
  2. The Transport Layer
  3. The Internet Layer
  4. The Link Layer

The Application Layer: The point of networking applications is to allow devices within the network to send different types of information to each other we can call this information data. The protocols in the application layer specify how data taken from an application can be used in the network. The most well known networking protocol is the Hypertext Transfer Protocol (HTTP), which we’ll get into in a bit.

The Transport Layer: The transport layer is below the application layer. It takes the data from the application layer and adds a “header” to the data, forming a segment. Because the protocols in this layer are responsible for ensuring the reliable transport of data, the header is typically comprised of a source port and a destination port, as well as some other things based on which protocol is being used. The two most common protocols in the transport layer are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). The difference:

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The Internet Layer: The internet layer takes the segment from the transport layer and adds another header to it forming an IP datagram/packet. Because Protocols in this layer specify how and where data ought to be shipped this header will typically include a source IP address and a destination IP address, which are essentially labels to individual devices in the network. IP is an acronym for the most used protocol in this layer, the Internet Protocol (IP).

The Link Layer: The link layer takes a packet from the internet layer adds a header and tail to it forming a frame. Because Protocols in this layer specify how to send the frames over the network, and connecting to a network requires both software and hardware, a frame  usually consists of a source and destination Media Access Control (MAC) address/physical address. The MAC address is linked to the hardware of a network adapter. In there words, an IP address is to software as a MAC address is to hardware. A frame contains everything needed to route the data to a destination.

The frame is then converted into bits where transmitted at the physical layer.

Whew! that was a lot and very technical. I found the following diagrams helpful for my own understanding as well as the RFC 1122 . Part 2. will be all about the application layer and specifically, HTTP.

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Images:

- http://www.cyberciti.biz/faq/key-differences-between-tcp-and-udp-protocols/\ - http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/learninglabs/9780134213736/graphics/30fig08.jpg\ - http://www.learn44.com/wp-content/uploads/2013/06/Protocol-Data-Unit-PDU-and-Layer-Addressing-in-Data-Encapsulation-Cisco-Inter-networking.jpg