Firewalls prevent unauthorized access to corporate networks from outside by only allowing certain connections and traffic. They inspect packets and filter them based on rules. Firewalls work with routers to relay data between networks and use different protocols like FTP. FTP uses two TCP connections - one for control and one for data transfer. When firewalls block standard FTP, passive mode is used to connect securely.

1. How Corporate
Firewalls Work
1. Firewall
2. FTP
3. Router
4. Packet

2. Firewall
The firewall is a system that prevents
it from invading the computer network
of the organization from the outside.
Or, it is a computer which such a
system was built in.

3. FTP
In the FTP, it operates while using two TCP
connections.
The connection is "Connection for control"
and "Connection for data transfer". When it
is not possible to connect it well in the
environment that uses firewall, a passive
mode is used.

4. Router
The machinery which relays data playing
with the router on a network in the other
networks. I analyze some protocols of
network layer (the class of the third) and the
transport layer (the class of the fourth) to say
by an OSI Reference Model and transfer.

5. Packet
The packet is the small unity of data added
the control information such as addresses of
the transmission of a message future to in
computer communication. I call a
communication method I divide data into a
packet, and to transmit and receive packet
transmission.

6. Reference
http://ja.wikipedia.org/
wiki/File_Transfer_Protocol
http://e-words.jp/