CCNA class notes.

Long before desktop computers with sophisticated graphical interfaces existed, people used text-based systems which were often just display terminals physically attached to a central computer. Once networks were available, people needed a way to remotely access the computer systems in the same manner that they did with the directly attached terminals.
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Telnet was developed to meet that need. Telnet dates back to the early 1970s and is among the oldest of the Application layer protocols and services in the TCP/IP suite. Telnet provides a standard method of emulating text-based terminal devices over the data network. Both the protocol itself and the client software that implements the protocol are commonly referred to as Telnet.
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Appropriately enough, a connection using Telnet is called a Virtual Terminal (VTY) session, or connection. Rather than using a physical device to connect to the server, Telnet uses software to create a virtual device that provides the same features of a terminal session with access to the server command line interface (CLI).
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To support Telnet client connections, the server runs a service called the Telnet daemon. A virtual terminal connection is established from an end device using a Telnet client application. Most operating systems include an Application layer Telnet client. On a Microsoft Windows PC, Telnet can be run from the command prompt. Other common terminal applications that run as Telnet clients are HyperTerminal, Minicom, and TeraTerm.
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Once a Telnet connection is established, users can perform any authorized function on the server, just as if they were using a command line session on the server itself. If authorized, they can start and stop processes, configure the device, and even shut down the system
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Telnet is a client/server protocol and it specifies how a VTY session is established and terminated. It also provides the syntax and order of the commands used to initiate the Telnet session, as well as control commands that can be issued during a session. Each Telnet command consists of at least two bytes. The first byte is a special character called the Interpret as Command (IAC) character. As its name implies, the IAC defines the next byte as a command rather than text.
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Some sample Telnet protocol commands include:
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Are You There (AYT) - Lets the user request that something appear on the terminal screen to indicate that the VTY session is active.
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Erase Line (EL) - Deletes all text from the current line.
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Interrupt Process (IP) - Suspends, interrupts, aborts, or terminates the process to which the Virtual Terminal is connected. For example, if a user started a program on the Telnet server via the VTY, he or she could send an IP command to stop the program.
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While the Telnet protocol supports user authentication, it does not support the transport of encrypted data. All data exchanged during a Telnet sessions is transported as plain text across the network. This means that the data can be intercepted and easily understood.
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If security is a concern, the Secure Shell (SSH) protocol offers an alternate and secure method for server access. SSH provides the structure for secure remote login and other secure network services. It also provides stronger authentication than Telnet and supports the transport of session data using encryption. As a best practice, network professionals should always use SSH in place of Telnet, whenever possible.
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Later in this course, we will use Telnet and SSH to access and configure network devices over the lab network.
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The Application layer is responsible for directly accessing the underlying processes that manage and deliver communication to the human network. This layer serves as the source and destination of communications across data networks.
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The Application layer applications, protocols, and services enable users to interact with the data network in a way that is meaningful and effective.
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Applications are computer programs with which the user interacts and which initiate the data transfer process at the user's request.
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Services are background programs that provide the connection between the Application layer and the lower layers of the networking model.
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Protocols provide a structure of agreed-upon rules and processes that ensure services running on one particular device can send and receive data from a range of different network devices.
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Delivery of data over the network can be requested from a server by a client, or between devices that operate in a peer-to-peer arrangement, where the client/server relationship is established according to which device is the source and destination at that time. Messages are exchanged between the Application layer services at each end device in accordance with the protocol specifications to establish and use these relationships.
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Protocols like HTTP, for example, support the delivery of web pages to end devices. SMTP/POP protocols support sending and receiving e-mail. SMB enables users to share files. DNS resolves the human legible names used to refer to network resources into numeric addresses usable by the network.
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The Application layer is responsible for directly accessing the underlying processes that manage and deliver communication to the human network. This layer serves as the source and destination of communications across data networks.
​
The Application layer applications, protocols, and services enable users to interact with the data network in a way that is meaningful and effective.
​
Applications are computer programs with which the user interacts and which initiate the data transfer process at the user's request.
​
Services are background programs that provide the connection between the Application layer and the lower layers of the networking model.
​
Protocols provide a structure of agreed-upon rules and processes that ensure services running on one particular device can send and receive data from a range of different network devices.
​
Delivery of data over the network can be requested from a server by a client, or between devices that operate in a peer-to-peer arrangement, where the client/server relationship is established according to which device is the source and destination at that time. Messages are exchanged between the Application layer services at each end device in accordance with the protocol specifications to establish and use these relationships.
​
Protocols like HTTP, for example, support the delivery of web pages to end devices. SMTP/POP protocols support sending and receiving e-mail. SMB enables users to share files. DNS resolves the human legible names used to refer to network resources into numeric addresses usable by the network..