Essential Network Ports and OSI Model Layers
When a Web client receives an IP address of a Web server, the client browser uses the IP address and port 80 to request Web services. This request is sent to the server using the Hypertext Transfer Protocol (HTTP). The FTP server allows a client to exchange files between devices and manage files remotely, sending management commands to start applications. FTP sessions are sent to the server using destination port 21 (data FTP 20). After opening the session, the server will use port 20 to transfer data files.
E-mail servers run server software that allows them to interact with clients and other mail servers via the network. SMTP (25): A client sends messages to its local SMTP mail server via port 25. POP3 is a mail protocol where a server that supports POP clients receives and stores messages for users on port 110. IMAP4 keeps the messages in the mailboxes on the server unless the user deletes them, using port 143. Configuring an email client involves specifying the POP3 or IMAP4 server name, SMTP server name, username, password, spam filters, and antivirus settings.
Instant Messaging (IM) Server: Runs locally on each computer and allows users to communicate or chat online in real-time, enabling the transfer of video files, music, and voice. PSTN: Public Switched Telephone Network. Telnet (23), SMTP (25), DNS (53), Client (67), Server (68), DHCP, TFTP (69), NBNS (137), SNMP (161), HTTPS (443)
OSI Model Layers
1. Network Access Layer: Processes routines for accessing the physical media.
2. Internet Layer: Defines the datagram and manages the routing of data.
3. Transport Layer: Provides end-to-end data delivery.
4. Application Layer: Includes the applications and processes using the network.
Advantages: Helps in the design of protocols, encourages competition, prevents changes in technology or the capabilities of a layer from affecting other layers above and below, and provides a common language to describe network features and capabilities.
Data:
- TCP (ports of origin and destination)
- IP (IP address of origin and destination)
- Ethernet (MAC address of origin and destination), upon receiving it in reverse.
Open Systems Interconnection (OSI) Model Layers:
Application Layer: Defines interfaces between communication network functions and software, providing standardized services such as file transfers between systems. Standardized data formats for user data that can be used between different types of systems. Encodes, encrypts, compresses, and decompresses data.
Session Layer: Establishes sessions and manages user dialogs, maintaining logical links between systems.
Transport Layer: Manages end-to-end message delivery across the network.
Network Layer: Routes packets according to unique addresses of network devices.
Data Link Layer: Defines procedures for using communication links and detects and corrects transmission errors in frames.
Physical Layer: Defines the physical means to send signals through the network, acting as an interface between devices and the network media, defining optical, electrical, and mechanical characteristics.