The four layers of the TCP/IP model are:

1. Application Layer
2. Transport Layer
3. Internet Layer
4. Network Interface Layer

IP (Internet Protocol) is responsible for addressing and routing packets between different networks. It provides a unique address (IP address) to each device on the network, allowing data to be sent and received across interconnected networks.

TCP (Transmission Control Protocol) is connection-oriented, reliable, and ensures ordered data delivery with error checking. UDP (User Datagram Protocol) is connectionless, faster, but less reliable, without guaranteed order or delivery of packets.

DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses and other network configuration parameters to devices on a network, simplifying network administration and avoiding IP address conflicts.

A switch operates at the data link layer (Layer 2) and forwards data between devices on the same network segment based on MAC addresses. A router operates at the network layer (Layer 3) and forwards data between different networks based on IP addresses.

A VLAN (Virtual Local Area Network) is a logical grouping of network devices that allows them to communicate as if they were on the same physical network segment, regardless of their physical location. Its primary purpose is to improve network performance, security, and manageability by segmenting a network into smaller, more manageable parts.

A firewall is a network security device that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It enhances network security by creating a barrier between trusted internal networks and untrusted external networks, preventing unauthorized access and potential threats.

Encryption in network security is used to protect sensitive data by converting it into a coded form that can only be deciphered with the correct decryption key. It ensures data confidentiality and integrity during transmission, making it difficult for unauthorized parties to intercept and understand the information.

The `ping` command is used to test the reachability of a host on an IP network and measure the round-trip time for messages sent from the originating host to a destination computer. It helps in identifying connectivity issues and latency problems between network devices.

The `traceroute` command shows the path that a packet takes from the source to the destination, listing all the routers (hops) along the way. It provides information about each hop, including IP addresses and response times, helping to identify where network delays or failures occur in the path.