Virtual Private Networks (VPNs) are network services that provide secure connections over the public internet or other potentially untrusted networks. VPNs use encryption and tunneling protocols to protect data integrity and confidentiality, ensuring that sensitive information can be transmitted securely. They are commonly used by businesses to securely connect remote employees to corporate networks or to interconnect geographically separated offices. VPNs can also be used by individuals to maintain privacy or bypass internet restrictions. There are various VPN protocols, including Point-to-Point Tunneling Protocol (PPTP), Layer 2 Tunneling Protocol with Internet Protocol Security (L2TP/IPsec), Secure Sockets Layer/Transport Layer Security (SSL/TLS), and WireGuard. Each protocol has its own advantages and drawbacks, with factors such as performance, ease of implementation, and security considerations influencing the choice of which protocol to use.

Quality of Service (QoS) is a network service that prioritizes different types of data traffic to ensure that more sensitive or latency-sensitive applications receive the required bandwidth and minimal delay or loss. Networks with limited resources or high congestion levels can benefit from QoS, as it helps minimize the impact of non-critical or less time-sensitive traffic on the performance of critical applications. Examples of applications that benefit from QoS include voice over IP (VoIP), video streaming, online gaming, and real-time collaboration tools. QoS mechanisms can be implemented at various levels, such as in routers, switches, or end devices. Techniques include traffic classification and marking (assigning priorities to different traffic types), traffic shaping (controlling the rate of data transmission), and traffic policing (enforcing bandwidth limits). Properly configured QoS can significantly enhance the performance and user experience of a network.

Simple Network Management Protocol (SNMP) is a standard protocol used to manage and monitor network devices, such as routers, switches, servers, and other IP-enabled devices. SNMP operates at the application layer, allowing administrators to keep track of network performance, troubleshoot issues, and manage configuration changes. SNMP uses a client-server model that relies on SNMP agents (devices) and management systems (applications). Network devices store management information in a database called the Management Information Base (MIB), which consists of hierarchical structures of managed objects identified by Object Identifiers (OID). SNMP performs operations using four key components: Get, Set, Trap, and Inform. SNMP supports three versions (SNMPv1, SNMPv2c, and SNMPv3), with SNMPv3 adding significant security enhancements, such as authentication and encryption.

Internet Group Management Protocol (IGMP) is a network-layer protocol used by IP hosts and adjacent routers to establish multicast group memberships. IGMP allows hosts to request multicast traffic from a router for specific multicast group addresses, enabling efficient distribution of network traffic to multiple recipients. IGMP offers a mechanism for routers to detect active multicast groups and determine which hosts are members of those groups, preventing unwanted traffic from being forwarded to hosts not interested in the multicast data. IGMP operates within a subnet and supports multicast forwarding over multiple hops via routing protocols like Protocol Independent Multicast (PIM) or Multicast OSPF (MOSPF). IGMP consists of three versions, IGMPv1, IGMPv2, and IGMPv3, with each subsequent version introducing improvements in speed, efficiency, and available features.

The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used to automate the assignment of IP addresses and other network configuration parameters to devices connected to a network. DHCP allows devices to join a network without manual configuration by a network administrator, significantly reducing the time and complexity required to maintain IP addressing schemes. The protocol operates on a client-server model wherein a DHCP server dynamically assigns IP addresses, subnet masks, default gateways, and other network parameters to clients. The DHCP process involves four main steps: Discovery, Offer, Request, and Acknowledgment. The communication between the DHCP client and server is based on DHCP messages, which are transported via User Datagram Protocol (UDP).

File Transfer Protocol (FTP) is a standard network protocol used to transfer files between a client and a server over a TCP-based network, such as the internet. FTP uses a client-server architecture, where the client initiates a connection to the server, typically through an FTP client application, and subsequently uploads or downloads files as needed. The protocol operates on two separate channels: the control channel (used for exchanging commands and responses) and the data channel (used for transferring file data). FTP connections can be established in either Active or Passive mode, which determines how the data channel is created. Active mode involves the server initiating the data connection to the client, while Passive mode has the client initiating the connection to the server. FTP uses TCP ports 20 and 21 for data and control connections, respectively.

Network Time Protocol (NTP) is a widely used protocol for synchronizing the clocks of computers and other devices within a network to a reference time source, such as an atomic clock or a GPS receiver. Accurate time synchronization is critical for various network services, including authentication, logging, billing, and event correlation. NTP operates on a hierarchical, client-server model, where clients synchronize their clocks with one or more NTP servers, which in turn synchronize with higher-tier servers or reference clocks. The protocol uses a complex set of algorithms to account for factors like network latency and utilizes the User Datagram Protocol (UDP) on port 123 for communication. NTP provides a high degree of accuracy, reducing time discrepancies to a few milliseconds or even microseconds in some cases.

Remote Desktop Protocol (RDP) is a protocol developed by Microsoft, allowing users to remotely access and manage a computer's desktop over a network connection. RDP transmits screen images from the remote computer to the client, while also transmitting keyboard and mouse inputs from the user. RDP is designed to work well over low bandwidth connections and is equipped with features such as clipboard sharing, printer redirection, and multiple monitor support. Remote Desktop Services, formerly known as Terminal Services, is a Windows Server feature that enables multiple users to connect and run applications on a single server, helping to save resources and simplify management.

Secure Shell (SSH) is a cryptographic network protocol used for secure data communication, remote command execution, and secure network services between two networked devices. Compared to its predecessor, Telnet, SSH provides a higher degree of security by encrypting data at the transport layer, ensuring confidentiality and integrity. In addition to encryption, SSH also implements public key authentication and strong passwords to verify the identity of communicating parties. SSH has various use cases, including remotely managing network devices, servers, and transferring files using protocols like SCP and SFTP. SSH is widely used in managing different types of systems and ensuring secure access in both local and remote scenarios.