Freecom Technologies FSG-3 manual Dhcp, Multiple internal but only one external IP address

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Freecom FSG-3 Storage Gateway

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-3 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 changing.

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 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 Translation 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.

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