Introductory Networking

An introduction to networking theory and basic networking tools

The OSI Model: An Overview

Notes

The OSI (Open Systems Interconnection) Model is a standardized model which we use to demonstrate the theory behind computer networking.

The OSI Model consists of 7 layers

Layer 7: Application

This layer of the OSI model essentially provides networking options to programs running on a computer. It works almost exclusively with applications, providing an interface for them to use in order to transmit data.

Layer 6: Presentation

The presentation layer translates the data into a standardized format, as well as handling any encryption, compression or other transformations to the data.

Layer 5: Session

It is responsible to setting up a connection with the other computer across the network. If a session can be established then it's the job of the session layer to maintain it, as well as co-operate with the session layer of the remote computer in order to synchronize communications.

Layer 4: Transport

Its first purpose is to choose the protocol over which the data is to be transmitted. The two most common protocols in the transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). With TCP the transmission is connection-based (Connection between the computers is established and maintained for the duration of the request). With UDP , packets of data are thrown at the receiving computer. With the protocol selected, the transport layer then divides the transmission up into bite-sized pieces (Over TCP these are called segments, over UDP they are called datagrams)

Layer 3: Network

The network layer is responsible for locating the destination of your request. At this stage we are working with what is referred to as Logical addressing (i.e. IP addresses) which are still software controlled. Logical addresses are used to provide order to networks , categorizing them and allowing us to properly sort them. The most common form of logical addressing is IPV4 format.

Layer 2: Data Link

The data link layer focuses on the physical addressing of the transmission. It receives a packet from the network layer and adds in a physical (MAC) address of the receiving endpoint. Inside every network enabled computer is a Network Interface Card (NIC) which comes with a unique MAC (Media Access Control) address to identify it.

Layer 1: Physical

The physical layer is right down to hardware of the computer. This is where the electrical pulses that make up data transfer over a network are sent and received. Its the job of the physical layer to covert the binary data of the transmission into signals and transmit them across the network, as well as receiving incoming signals and converting them back into binary data.

Questions

Encapsulation

Notes

As data is passed down each layer of the model, more information containing details specific to the layer in question is added on to the start of the transmission.

Encapsulation is the process by which data can be sent from one computer to another

When the message is received by the second computer, it reverses the process. This is known as de-encapsulation.

Questions

The TCP/IP Model

Notes

It serves as the basis for real-world networking. The TCP/IP model consists of four layers: Application, Transport, Internet and Network Interface.

The OSI Model and the TCP/IP model match up like this:

The process of forming a stable connection between two computers is called the three-way handshake.

Questions

Networking Tools

Ping

The ping command is used when we want to test whether a connection to a remote resource is possible.

Traceroute

Traceroute can be used to map the path your request takes as it heads to the target machine. On Windows it is Tracert.

WHOIS

It essentially allows you to query who a domain name is registered to.

Dig

Dig gives us the TTL (Time To Live) of a queried DNS record.