Network Address Translation - Snom 4S Administrator's Manual

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the destinations that are provided with the 3xx response and puts them on
the list. This is done using the probability of the underlying registrations,
and redirected contacts may have an even lower probability.
If the 3xx response for the only pending request, the proxy
passes the response through to the request client. The client itself then
takes care of the redirection. This is important in cases where a call from
a PSTN gateway comes in and should be redirected to a PSTN number.
The gateway then does not have to pass the call through the proxy, it
can immediately redirect the call on the PSTN level. This does not work
however, if the call is redirected to more than one PSTN number, in which
case the call will have to go through VoIP.
In many environments, users are registered with a high
probability and the mailbox is registered as well, but with a low value.
Even if the phone is switched off, the proxy will redirect the call to the
mailbox after a timeout.

Network Address Translation

One of the biggest problems with the Internet version 4 is that
it has a limited address range. IP V4 defines 32 bits for addresses, which
were distributed according to a geographic scheme at a time when
the success of the Internet protocol was hard to envision. Regions like
Europe or Asia in particular, did not receive enough numbers to connect
every network element with its own IP address. Because of this, in many
installations several computer and network elements have to share one IP
address. They do this by using different ports of the address. An IP packet
may go to one of 65,535 ports, making up roughly 16 bits and extending
the internet address to about 48 bits.
The computer that owns the IP address is called the network
address translation (NAT) gateway. This computer has one "real" IP
address (called public IP address) and a private IP address that is visible
only to the computers that share the public IP address in a private network.
The internet society has defined a specific address space that is used as
private IP addresses (192.168.x.x, 10.x.x.x and 172.[16-31].x.x).
The NAT gateway keeps a list of ports and associated private
IP addresses. Whenever a packet arrives at the NAT from the public
Internet, the NAT gateway looks into the list and forwards the packet to
the associated computer and port. When a packet arrives from a private
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