Network Address Translation - Snom 4S User Manual

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 address, the NAT
forwards it to the Internet and keeps an association between the private
address and the port used for the forwarding. In this way the association
table can be set up. There are hundreds of other ways to set up and maintain
the NAT table. The mechanism is often combined with a firewall that inspects
packets during the forwarding process.
The Internet Protocol V6 solves the NAT problem in a different way.
They simply use many more bits to identify a host, but this is much harder to
implement and maintain, so NAT is still more popular.
50 • General Concepts