A MAC (Media Access Control) address is primarily used for identifying network devices at the data link layer. It's a unique identifier assigned to network interface cards (NICs) and is essential for data frames to be properly delivered within a local network.

John should implement the IMAP (Internet Message Access Protocol). IMAP allows users to access their emails from multiple devices, and actions like deleting or reading an email on one device are synchronized across all devices. It's suitable for scenarios where users need access to their emails from various locations and devices.

A recursive resolver is responsible for resolving DNS queries on behalf of clients. It performs the entire process of DNS resolution, starting from querying authoritative name servers to returning the IP address associated with a domain.

RIP (Routing Information Protocol) would be the least suitable for large-scale networks due to its limitation in hop counts. RIP uses hop count as a metric, which can lead to routing loops and suboptimal paths in large networks. More advanced protocols like OSPF, EIGRP, and BGP are better suited for large-scale environments.

FTP (File Transfer Protocol) data transfer is commonly secured using TLS (Transport Layer Security) encryption. This encryption ensures that data sent over FTP is protected from eavesdropping and tampering, making it a safer choice for transferring sensitive information.

When a user connects to a remote network in a way that makes it seem as if they are directly connected to the network's private internal system, they are using a VPN (Virtual Private Network). A VPN creates a secure, encrypted tunnel over an untrusted network (like the internet) to provide remote access with the appearance of being locally connected.

ARP (Address Resolution Protocol) is used to resolve an IP address into a MAC address within a local network. When a device needs to communicate with another device on the same local network, it uses ARP to find the MAC address associated with the destination IP address.

OSPF (Open Shortest Path First) is the most appropriate routing protocol in this case. OSPF is specifically designed for large internal networks and offers scalability and efficient handling of topology changes within areas. It's widely used for internal routing within organizations.

The designated router (DR) in OSPF (Open Shortest Path First) is responsible for advertising Link State Advertisements (LSAs) to other routers within the same OSPF area. It helps reduce the flooding of LSAs and optimizes the OSPF network by centralizing LSA update distribution.

TCP (Transmission Control Protocol) - TCP is responsible for ensuring that data packets are delivered in the correct sequence and without errors. It provides error-checking, sequencing, and flow control during data transmission, making it a reliable choice for applications that require data integrity.