User's Manual

SPEEDLAN 9000 Installation and Operation User Guide
Basics of IP Addressing 5-9
What is NAT?
Network Address Translation (NAT) is the conversion of an Internet Protocol address (IP address)
used within one network to a different IP address within another network. One network is designated
the inside network and the other is the outside network.
Network Address Translation (NAT) occurs when there is a translation among an Internet Protocol (IP
address) used within one network (designated as inside network) to a different IP addresses within
another network (designated as outside network). Network Address Translators (NATs) allow
companies to decrease the number of global IP addresses. This enables companies to communicate
with other devices on the Internet with a single global IP address (or more than one IP address).
For example, a company can provide its clients with one IP address, allowing access to the
company's firewall only. This IP address is not a "real" address on the company's internal network, but
it is successfully translated to the correct IP location through NAT (i.e., NAT unit/router). Therefore,
the company controls access through firewalls and provides multiple IP addresses to outside
customers without excessive limited resources, or "global" Internet IP protocols.
NAPT
What differentiates NAPT from NAT? NAPT (or Network Address Port Translation) not only translates
the IP address but also the transport layer port. Thus, if an inbound packet addressed to port 80 on
the NAPT device would be translated and passed to the private network's Web server. Without port
translation, the NAT device has no means of knowing which host in the private network can pass
packets to other devices. For an example see, Diagram of Incoming NAT, page 5-11.