Owner's Manual

Protecting Your Network
42
N300 Wireless Gigabit ADSL2+ Modem Router DGN3500
Figure 21.
For any traffic attempting to pass through the firewall, the packet information is subjected to
the rules in the order shown in the Rules table, beginning at the top and proceeding to the
default rules at the bottom. In some cases, the order of precedence of two or more rules
might be important in determining the disposition of a packet. The Move button allows you to
relocate a defined rule to a new position in the table.
The Firewall Rules screen also lets you open or close AOL or MSN Instant Messenger ports:
1 Under Inst
ant Messaging (IM) Ports, select a radio button:
• Close IM Port
s. Specifies to disable instant messaging traffic.
• Ope
n IM Ports. Specifies to enable instant messaging traffic. IM ports are open by
default.
2 Click Apply to
save your changes.
Services
Services are functions performed by server computers at the request of client computers. For
example, Web servers serve Web pages, time servers serve time and date information, and
game hosts serve data about other players’ moves. When a computer on the Internet sends a
request for service to a server computer, the requested service is identified by a service or
port number. This number appears as the destination port number in the transmitted IP
packets. For example, a packet that is sent with destination port number 80 is an HTTP (Web
server) request.
The service numbers for many common protocols are defined by the Internet Engineering
T
ask Force (IETF) and published in RFC1700, “Assigned Numbers.” Service numbers for
other applications are typically chosen from the range 1024 to 65535 by the authors of the
application.