Firewall software controls connections between an internal computer network and external networks by filtering packet and application data. Firewalls can operate at different layers, with packet filters at the network and transport layers, circuit gateways at the transport layer, and application gateways at the application layer. The FTP protocol is used to transfer files between a client and server over a network, and is commonly used to upload web pages and files to a server or download files from an FTP server. Routers interconnect different networks and exchange IP packets between networks at the network layer, while also providing additional functions.

1. How Corporate Firewalls Work
・Firewall
・ FTP
・Packet
・Router

2. Firewall
Firewall is software that control connect
computer network with outside of
network.
Firewall can be divided into some types
by OSI.
Packet filters type works on network
layer and transport layer. Circuit level
gateway type works on transport layer.
This type is added to hardware as part of
the function. Application gateway type
works on application layer.

3. FTP
The FTP is abbreviation of File Transfer Protocol. It is a
protocol to forward the file on the network. That exists
since the Internet initial, it is one of the protocols often still
use by the Internet.
On the protocol, the file transfer between arbitrary hosts
can be done. But, it is used forwarding between the client
and the server connecting the usually.
As a usage:
● Uploading of HTML file and picture file, etc. to web
server.
● Download of file from FTP file server to client.
It is possible to download it also with the browser software.
The FTP client software or the CUI command are need for
uploading.

4. Packet
The collected small thing of the data to which
control information, including the address of a
transmission place etc., was added in online
communications. The communication method
which divides, transmits and receives data to a
packet is called packet communication. It is lost
that an intermediate circuit is occupied by
communication between some 2 points, and a
communication line can be efficiently used by
dividing, transmitting and receiving data to many
packets. Moreover, since channel selection can be
performed flexibly, even if an obstacle comes out
in part, there is also an advantage that other
circuits can be substituted.

5. Router
In a computer network , it is a communication facility which carries out
interconnection of between two or more different network . Since tcp/ip
came to be used for a communications protocol , it spread .
Outline
A router is a communication facility whic carries out interconnection of
between networks,and if it says by the OSI basic reference model whic
consists of seven layers which classified the function of the computer
network,it will bear connection from the 1st layer "physical layer"to the
3rd
layer "network layer."Although a basic function is equipment whic enables
it
to exchange the IP packet between each network,in addition to
foundations,various addition functions are realized in practice.

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