SSH is a network protocol that allows secure remote access to a computer or server. It operates on port 22 by default. This means that if you want to establish an SSH connection to a device, you need to specify port 22 as the destination port. The other port numbers mentioned (21, 25, and 110) are associated with different protocols such as FTP, SMTP, and POP3 respectively, but they are not used for SSH connections. Therefore, the correct answer is 22.