Surveon NVR7312(2U), Руководство пользователя

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3.4. Port Forwarding
Port forwarding is a name given to the combined technique of:
1. Translating the address and/or port number of a packet to a new destination.
2. Possibly accepting such packet(s) in a packet filter (firewall).
3. Forwarding the packet according to the routing table.
To illustrate its concept, two computers on the Internet that communicate with each
other using TCP/IP or UDP/IP protocols(though the process is not limited to these)
utilize ports to identify the opposite connection points of each other where the data
packets supposed to go to. In order to communicate, each computer knows the port of
another computer (in addition to IP address) and sends the data to that port. Port
forwarding forwards these ports in such a way that when one computer sends data to
the specific port of another computer, the data is actually sent to a different port.
This allows remote computers to connect to a specific computer or service within a
private LAN.
In a typical residential network, nodes obtain Internet access through a DSL or cable
modem connected to a router or network address translator (NAT/NAPT). Hosts on the
private network are connected to an Ethernet switch or communicate via a wireless
LAN. The NAT device's external interface is configured with a public IP address. The
computers behind the router, on the other hand, are invisible to hosts on the Internet
as they each communicate only with a private IP address.
When configuring port forwarding, the network administrator sets aside one port
number on the gateway for the exclusive use of communicating with a service in the
private network, located on a specific host. External hosts must know this port number
and the address of the gateway to communicate with the network-internal service.
When used on gateway devices, a port forward may be implemented with a single rule
to translate the destination address and port. The source address and port are, in this
case, left unchanged. When used on machines that are not the default gateway of the
network, the source address must be changed to be the address of the translating
machine, or packets will bypass the translator and the connection will fail.