Secure Shell (SSH)

Correct Answer: Edit the 'AllowUsers' and/or 'AllowGroups' parameters in the SSH config file.

To limit access to a server's SSH connection to specific IP addresses, a company should 'Edit the 'AllowUsers' and/or 'AllowGroups' parameters in the SSH config file'. This allows them to specify which users or groups from which IP addresses are allowed to authenticate to the SSH server. 'Enable port forwarding for specific IP addresses' is not correct because port forwarding alone doesn't limit the access to specific IPs. 'Configure the SSH server to use a non-standard port' is also not correct because changing the port number doesn't restrict access to specific IPs. Lastly, 'Disabling the 'PermitRootLogin' parameter' is incorrect because it only prevents root login, not IP specific access.

Correct Answer: SFTP

SFTP is the correct protocol for transferring large files over an encrypted SSH connection. SFTP stands for Secure File Transfer Protocol, and it is built on the SSH protocol, providing both encryption and large file support. FTP, while capable of handling large file transfers, does not operate over an encrypted connection and hence is not suitable for secure file transfer. SSH is not a file transfer protocol, it's a network protocol primarily used for secure connection into remote servers. SCP (Secure Copy Protocol) could also be used for file transfers over an SSH connection, however, it is not as flexible or versatile as SFTP when dealing file manipulation.

Correct Answer: SSH public-private key authentication

The best security measure to prevent unauthorized access via SSH is 'SSH public-private key authentication'. This method uses a pair of cryptographic keys that are mathematically linked. The private key is kept secret and secure by the user, while the public key can be widely distributed. Someone who has the public key can encrypt a message, but only the private key can decrypt it. This is more secure than 'Password-based authentication' which is susceptible to brute force attacks and other hacking techniques. 'Using a different SSH port' could potentially add a slight layer of obscurity but does not solve the fundamental problem of secure authentication. Lastly, 'Disabling SSH access' would indeed prevent unauthorized SSH access, but it would also prevent authorized SSH access, rendering all SSH functionalities useless. Therefore, it's not a practical solution.