
User Guide 5
Introduction to Network Security
• World Wide Web access uses Hypertext Transfer Protocol (HTTP)
• E-mail uses Simple Mail Transfer Protocol (SMTP)
• File transfer uses File Transfer Protocol (FTP)
• Changing a domain name to an Internet address uses Domain Name Service (DNS)
• Remote terminal access uses Telnet or SSH (Secure Shell)
Some services are necessary, but each service you add to your security policy can also add a security
risk. To send and receive data, you must “open a door” in your computer, which puts your network at
risk. Attackers can use open access of a service to try to get into a network. We recommend that you
only add services that are necessary for your business.
Ports
Usually, a port is a connection point where you use a socket and a plug to connect two devices. Com-
puters also have ports that are not physical locations. These ports are where programs transmit data.
Some protocols, such as HTTP, have ports with assigned numbers. For example, most computers trans-
mit e-mail on port 25 because the SMTP protocol is assigned to port 25. Other programs are assigned
port numbers dynamically for each connection. The IANA (Internet Assigned Numbers Authority) keeps
a list of well known ports. You can see this list at www.iana.org/assignments/port-numbers.
Most services are given a port number in the range from 0 to 1024, but possible port numbers range
from 0 to 65535.
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