What ports should I unblock on a firewall?

Before continuing with this article, be aware that it's more
geared toward advanced users who can change rules on their
firewall.  Fidelity Communications can not be held responsible
for changes to your firewall settings.

There are two major types of protocols in the Internet protocol
(IP) used commonly today. TCP and UDP. TCP is designed for
higher reliability, whereas UDP is designed for faster transmissions.
TCP ports use a "three way handshake" to establish connections,
whereas UDP ports are just sent with no connection made. Each
port is assigned a number (0-65535 UDP and TCP). Each of these
ports is used by a different program. The first 1024 (0-1023) ports
are reserved for use by certain programs.

To have a strong firewalling policy, you should block everything
except what you need instead of blocking certain things that
you know are bad. Here are some ports related to some
services commonly used on the internet:

TCP Ports:

* 80 - HTTP (For loading web pages)
* 21 - FTP (Required for some downloads)
* 25 - SMTP (For sending e-mail)
* 110 - POP3 (For receiving e-mail)
* 443 - HTTPS (Secure web pages)
* 1863 - MSN Messenger (Popular instant messaging client)
* 5050 - Yahoo Messenger (Another popular instant messaging client)
* 5190 - ICQ (Yet another popular instant messaging client)

UDP Ports:

* 53 - DNS (Translates .coms into IP addresses) Note: this is a
critical service most programs that connect to the internet use this port.

Note: There are many other ports used by web programs. For
a list of these ports, see the vendor's documentation or this
page for a list of registered ports: www.iana.org/assignments/port-numbers