Appendix A : Network Address Translation - Dynamix UM-A User Manual

Appendix A : Network Address Translation

Network Address Translation (NAT) translates the IP address a network (LAN) to a different IP
address known by another network (WAN). This gives an outside network the ability to distinguish and
communicate with a device on the inside network, as the inside network has a private set of IP
addresses assigned by the DHCP server, which are not know to the outside network.
The rise of NAT and increasing use of NAT come from several factors.
World shortage of IP Addresses: Public IP addresses need to be used in the public domain.
However, the limited supply of public IP addresses cannot satisfy the increasing demand.
NAT allows multiple IP nodes in the private domain to share one public IP address. This
conserves the pool of public IP address, and makes private IP addresses reusable in other
private domains.
Privacy / Security: Concern in privacy and security arises when exposing IP addresses in a
private network to the public domain. NAT automatically provides firewall-style protection
by only allowing connections originated from the private network and not allowing attackers
on the public domain to distinguish individual IP addresses of computers internal to the
network.
Administrating external network topology changes: Without NAT, when the network
topology of the public domain changes, the address assignment for the local domain would
be forced to change accordingly. NAT separates the private network from the public domain.
Thus, changes of public domain network topology can be hidden from users within the
private domain.
NAT operation is based on where the traffic is initiated instead of the physical packet direction.
Outbound sessions are initiated from the private network accessing the external network. For example,
an FTP session initiated from a host in the private network to access the FTP server through the internet
is considered an outbound session. This session includes bi-directional packet exchange. The primary
NAT function allows outbound sessions so that hosts in a private network can transparently access the
external network.
Inbound sessions are initiated from the external network accessing the private network. For example,
an FTP session initiated by a host from the external network to access the FTP server residing in the
private network is considered an inbound session. NAT usually blocks all inbound sessions. Various
implementations may be added to extend the NAT function and enable selective inbound sessions to
allow access to local hosts from outside networks.
4/1 Port ADSL Router P 125