126
Freecom Storage Gateway (FSG)
GB
DHCP
First, most users today have a dynamically given IP address or DHCP address from their
provider. This means that the IP address given to you can change over time. This allows
the provider to use the IP address for more then one person or device, since people are
often online at different times. Optimal usage of IP addresses is guaranteed.
Your FSG gives its internal addresses to your computers in the same way. For the DNS
(Domain Name System) it is difficult to translate your domain name
"www.yourname.sharemydisk.com" to your IP address if your IP address keeps chang-
ing.
The system needs to be updated each time your IP address changes. Hence Dynamic
DNS (DDNS) programs. These programs check the latest IP address and update the DNS
system accordingly.
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 connec-
tion. 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.
