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NAT (Network address translation)
When a network device with a private IP address wants to send information via the Internet, it
must do so using a router that supports NAT. Using this technique, the router can translate a
private IP address into a public IP address without the sending host’s knowledge.
Port forwarding
To access cameras that are located on a private LAN via the Internet, the public IP address of the
router should be used together with the corresponding port number for the network camera/video
encoder on the private network.
Since a web service via HTTP is typically mapped to port 80, what happens then when there are
several network cameras/video encoders using port 80 for HTTP in a private network? Instead of
changing the default HTTP port number for each network video product, a router can be configured
to associate a unique HTTP port number to a particular network video product’s IP address and
default HTTP port. This is a process called port forwarding.
Port forwarding works as follows. Incoming data packets reach the router via the router’s public
(external) IP address and a specific port number. The router is configured to forward any data
coming into a predefined port number to a specific device on the private network side of the
router. The router then replaces the sender’s address with its own private (internal) IP address.
To a receiving client, it looks like the packets originated from the router. The reverse happens with
outgoing data packets. The router replaces the private IP address of the source device with the
router’s public IP address before the data is sent out over the Internet.
Figure 9.2a Thanks to port forwarding in the router, network cameras with private IP addresses on a local network
can be accessed over the Internet. In this illustration, the router knows to forward data (request) coming into port
8032 to a network camera with a private IP address of 192.168.10.13 port 80. The network camera can then begin
to send video.
193.24.171.247
Router
Port mapping in the router
External IP address External port Internal IP address Internal port
of router of network device
193.24.171.247 8028 192.168.10.11 80
193.24.171.247 8030 192.168.10.12 80
193.24.171.247 8032 192.168.10.13 80
192.168.10.11
Port 80
192.168.10.12
Port 80
192.168.10.13
Port 80
URL: http://
193.24.171.247:8032
HTTP Request
CHAPTER 9 - NETWORk TECHNOLOGIES
