What is NAT?

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NAT stands for Network Address Translation. It is proposed and described in RFC-1631 and is used for solving the public IP address depletion problem. At present, many Routers support this NAT function. The NAT box can "translate" the local IP addresses to global address and vice versa. In other words, it is a method of connecting multiple computers to the Internet (or any other IP network) using one public IP address.

What is "Private Network"?

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Cited from RFC1918: Address Allocation for Private Internets.

Hosts within enterprises that use IP can be partitioned into three categories:
Category 1: hosts that do not require access to hosts in other enterprises or the Internet at large; hosts within this category may use IP addresses that are unambiguous within an enterprise, but may be ambiguous between enterprises.
Category 2: hosts that need access to a limited set of outside services (e.g., E-mail, FTP, net news, remote login), which can be handled by mediating gateways (e.g., application layer gateways). For many hosts in this category an unrestricted external access (provided via IP connectivity) may be unnecessary and even undesirable for privacy/security reasons. Just like hosts within the first category, such hosts may use IP addresses that are unambiguous within an enterprise, but may be ambiguous between enterprises.
Category 3: hosts that need network layer access outside the enterprise (provided via IP connectivity); hosts in the last category require IP addresses that are globally unambiguous.

We will refer to the hosts in the first and second categories as "private". We will refer to the hosts in the third category as "public".

If it is necessary for a device on a private network to communicate with other networks it's necessary for a "mediating gateway" to ensure that the outside network is presented with an address that is "real" (or publicly-reachable) so that routers allow the communication. Typically this gateway will be a Network address translation (NAT) device or a proxy server.

What is "Private IP Address Space"?

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Cited from RFC1918: Address Allocation for Private Internets

The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets.

You can select one of the IP address listed above. For example, you can use the 192.168.100.x / 255.255.255.0 for your private use.

In Internet terminology, a private network is a network that uses RFC 1918 private IP address space. Computers may be allocated addresses from this address space when it's necessary for them to communicate with other computing devices on an internal (non-Internet) network but not directly with the Internet.

How can I find the IP address of my computer?

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Take Windows 2000 or XP O/S for an example.
Go to "Start" >> select "Execute" >> type "cmd" into the blank column >> press the "Enter" key >> type "ipconfig" and then press "Enter" key >> then you will see the following screen with information of IP address, subnet mask and default gateway.

I want to share a cable or DSL internet connection with camera and PCs.

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The Broadband router will be a better solution. This will allow you to share multiple computers and camera with a single IP address from your CABLE / DSL provider.

What is PPPoE?

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PPPoE stands for Point-to-Point Protocol over Ethernet. It is a non-standard method of connecting to your ISP to gain a dynamic IP address. It relies upon a software client that is provided by the ISP. A dynamic IP address is required to gain a connection to the Internet.

What is DHCP?

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Cited from Wikipedia

In the context of computer networking, Dynamic Host Configuration Protocol (DHCP, currently implemented as DHCPv6) is a client-server networking protocol. A DHCP server provides configuration parameters specific to the DHCP client host requesting, generally, information required by the client host to participate on an IP network. DHCP also provides a mechanism for allocation of IP addresses to client hosts.

What is DDNS?

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DDNS stands for Dynamic DNS, is a service for transforming dynamic IP to corresponding to a specific "Hostname". There are many websites providing free DDNS services. For example, http://www.dyndns.org.

Cited from Wikipedia: Dynamic DNS is a system for allowing an Internet domain name to be assigned to a computer with a varying IP address. This makes it possible for other sites on the Internet to establish connections to the machine without needing to track the IP address itself.

Dynamic DNS service is provided on a large scale by various DNS hosting services, which retain the current addresses in a database and provide a "client" program to the user who will send an update to the service whenever the server's IP address has changed. Many routers and other networking components contain a feature such as this in their firmware.