A computer must have an IP (Internet Protocol) address to allow it to communicate with other computers. An IP address is a 32 bit number that identifies the location of your computer on a network. It looks something like 192.168.0.1.
A NAT (network address translation) router coordinates traffic between the local network and the Internet. A single, unique IP address is used to represent a group of computers. NAT was invented by Cisco.
A LAN (Local Area Network) uses IP addresses internally. Most of the network traffic in a LAN is local, so it doesn't travel outside the internal network. For this local communication, NAT is not required. Usually any computer in a LAN must use Network Address Translation to communicate outside of the local network.
When an Internet communication transaction is initiated by a computer inside a LAN, the NAT router directs the communication to the Internet, but if an Internet communication transaction is initiated from outside the LAN, the NAT router may not have the information necessary to direct the transaction to the correct computer. This is where "port forwarding" becomes relevant. The router can be configured such that any communication received on a certain port will automatically be routed to the assigned computer.
Accessing the Network DVR
The result is that in order to access a network DVR from inside the local network, one enters the DVR's IP address into the browser address window. If the DVR has been configured for a port other than 80 (80 is the default port for HTTP), for example "8484", then the IP address will be followed by ":8484". The DVR in the picture would have the address "http://192.168.0.4:8484". However, to access the DVR from outside the local area network, one will use the address of the NAT router, and if port forwarding has been properly configured, the communication will be routed to the DVR. In this case the DVR in the picture would have the address "http://XXX.YYY.Z.N:8484".
Static and Dynamic IP Addresses
A static IP address is a number that is assigned to a computer by an Internet service provider (ISP) for a permanent address on the Internet. It would be more simple if every computer that connected to the Internet had its own static IP address, but when the Internet was first developed, the architects didn't anticipate the need for billions of IP addresses. Consequently, Internet service providers limit the number of static IP addresses they allocate. Most customers receive a temporary IP address which may change frequently. The temporary IP address is called a dynamic IP address.
This can create a problem for accessing a DVR with a customer who has a dynamic IP address. It would be comparable to having a phone number that changed. If one called that phone number, it would go to the wrong customer. The simplest solution is to request a static IP address from the ISP. An alternative solution is to register with DYNDNS.org so that a web address can be assigned to your network. Many NAT routers can be configured to automatically notify the system when the IP changes, so that the web address stays current with your dynamic IP address.