Configuring the DSL Router
3-6
6371-A2-GB20-10
August 2000
Network Address Translation (NAT)
Network Address Translation is used when a private network’s internal IP
addresses cannot be used outside the private network. The IP addresses may be
restricted for privacy reasons or they may not be valid public IP addresses.
The DSL router provides NAT as described in
RFC 1631
The
IP Network Address
Translator (NAT)
.
NAT allows the private (local) hosts to transparently access
public (global) external IP addresses.
Two variations of traditional NAT are supported:
H Basic NAT
H Network Address Port Translation (NAPT)
NOTE:
Basic NAT and NAPT cannot be enabled at the same time.
Basic NAT
Basic NAT allows hosts in a private network to transparently access the external
network by using a block of public addresses. Static mapping enables access to
selected local hosts from the outside. Basic NAT is often used in a large
organization with a large network setup for internal use and the need for
occasional external access.
Basic NAT provides a one-to-one mapping by translating a range of assigned
public IP addresses to a similar-sized pool of private addresses (typically from the
10.
x.x.x
address space). Each local host currently communicating with a external
host appears to have an unique IP address. Up to 256 IP addresses can be
allocated for use with Basic NAT.
Network Address Port Translation (NAPT)
NAPT allows multiple clients in a local network to simultaneously access remote
networks using a single IP address. This benefits telecommuters and SOHO
(Small Office/Home Office) users that have multiple clients in an office running
TCP/UDP applications. NAPT is sometimes referred to as PAT (Port Address
Translation).
NAPT provides a many-to-one mapping and uses one public address to interface
numerous private users to an external network. All hosts on the global side view
all hosts on the local side as one Internet host. The local hosts continue to use
their corporate or private addresses. When the hosts are communicating with
each other, the translation is based on the IP address and the IP port numbers
used by TCP/IP applications.
