Efficient Networks SpeedStream 5600 Series - Manual (Page 35)

34 Technical Concepts Efficient Networks, Inc.
unique within the private address space, but two private address spaces are
not guaranteed unique.
Use of private address spaces has some disadvantages including the need
to re-address any host that must change from a private address to a public
address. Plus the privately addressed hosts are unable to communicate with
all hosts in an internet. These problems can be handled by the use of
Network Address Port Translation (NAPT).
NAPT is an extension to Network Address Translation (NAT). With NAT,
a network address translator (the router, in this case) sits between an
organization’s network and the Internet, or between two organization’s
networks and translates IP addresses from private internal addresses to
globally unique external addresses. NAPT, however, allows many network
addresses and their TCP/UDP ports to be translated to a single network
address and its TCP/UDP ports. With NAPT, a few of your internal hosts
can share a single public address. When a host needs to access the
Internet, the router will translate an address for it. When packets from the
host are sent to the Internet, the router replaces the internal address with
the external address. When packets come back for that address, the router
reverses the substitution.