A firewall is used to selectively block both inbound and outbound network communications. Firewalls range from simple Windows based software to complex UNIX based software to small hardware devices arranged between the physical wiring of an internal network and an external network (the internet).

     In the non-computer industries, a firewall is an actual wall designed to stop a fire from spreading in a building complex such as apartments and town homes. In the computer world, a firewall is a virtual wall designed to stop the fire of the internet (hackers) from coming inside a secure network.

     Most firewalls are configured by a network administrator. The configuration process most often involves an order of filters or circumstances that must be met to allow network traffic to pass. These filters are based on multiple characteristics such as the traffics originating IP address, its destination IP address, its originating port, its destination port, and its connection protocol (TCP/UDP/ICMP). For example, to send email to aol.com, a network connection would have your IP address as its originating address, aol.com's SMTP server's IP address as it's destination address, and TCP port 25 as its connection socket. Had this data transaction not met the rules of a firewall between you and the aol.com's SMTP server, the data communications would have been dropped, effectively blocking the transaction.

FTP.........: TCP<-21
           .: TCP->20
TELNET......: TCP->23
SMTP........: TCP->25
DNS.........: UDP->53
           .: UDP<-53
POP3........: TCP->110

PING........: ICMP
TRACEROUTE .: ICMP


FTP Daemons:
	Monitor incoming TCP 21, output to TCP 20.
	FTPD TCP21 <- FTPC TCP?
	FTPD TCP20 -> FTPC TCP?