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Multiple internal but only one external IP address
A second way to overcome this is to have a single device act on behalf of several other
devices. Routers are typical examples. You have several PCs connect to the router, but
only the router to the Internet. From an internet perspective, there is only one address.
This scheme offers the additional benefit of simple firewall-like protection because the
internal LAN addresses are not available to the Internet through the translated connection.
All incoming inquiries are filtered out by the router. This filtering can prevent intruders
from probing your system. However, using port forwarding, you can allow one PC (for
example, a Web server) on your local network to be accessible to outside users.
This works like a receptionist at a large office. Nobody knows your number; they all call
the main number of the office and are then forwarded by the receptionist to you.
So how does the receptionist know what number to forward the call too? The caller tells
the receptionist what name to look for and the receptionist has a listof names and numbers
at her disposal so she can translate the name to the phone extension she needs.
On the Internet, it works exactly the same. NAT (RFC 1631) or Network Address Transla-
tion allows a single device, such as a router, to act as an agent between the Internet (or
"public network") and a local (or "private") network. This means that only a single, unique
IP address is required to represent an entire group of computers.
Network Address Translation
When you visit a website via your computer, your router, or more specifically your NAT, remem-
bers which computer on the internal network asked for the information. When the information
(in this case the website you asked for) comes back, the router knows which computer on the
internal network to send the information to. This is Network Address Translation or NAT.
Developed by Cisco, Network Address Translation is used by a device (firewall, router or com-
puter) that sits between an internal network and the rest of the world. For more information
about IP address translation, refer to RFC 1631, The IP Network Address Translator (NAT).
Hosting a server
What If I want to host a server?
For others to be able to view your pictures on your computer, you need to run some
type of service or server. A web server such as apache will allow others to securely watch
the pictures or files on your site. But how does the NAT of your router know this time to
which computer to forward the information?
Since the request comes from the internet, most routers are setup to disallow any traffic going into
the internal network. (Firewall protection) In this case, since we want people to be able to see our
pictures, we want the router to forward this information. We have to tell the router to do this.
Someone calling from the internet asks your router for a certain "port". This is the same as
someone calling the receptionist and requesting a certain person by name. Ports are dis-
played as numbers but invariably map to pre described services. For instance, when someone
requests a web page, they always ask for the service on port 80, the web server. Through NAT,
it is possible to "map" a certain request for a service (a request for a port) to a certain PC on the
internal network.