Source NAT is typically used by internal users to access
the Internet; the source address is translated and thereby kept
private. There are three types of source NAT:
Dynamic IP and Port (DIPP)
—Allows multiple hosts
to have their source IP addresses translated to the same public
IP address with different port numbers. The dynamic translation
is to the next available address in the NAT address pool, which
you configure as a
be to an IP address, range of addresses, a subnet, or a combination
As an alternative to using the next address in
the NAT address pool, DIPP allows you to specify the address of
itself. The advantage of specifying
the interface in the NAT rule is that the NAT rule will be automatically
updated to use any address subsequently acquired by the interface.
DIPP is sometimes referred to as interface-based NAT or network address
port translation (NAPT).
—Allows the one-to-one, dynamic translation
of a source IP address only (no port number) to the next available
address in the NAT address pool. The size of the NAT pool should
be equal to the number of internal hosts that require address translations.
By default, if the source address pool is larger than the NAT address
pool and eventually all of the NAT addresses are allocated, new connections
that need address translation are dropped. To override this default behavior,
Advanced (Dynamic IP/Port Fallback)
enable use of DIPP addresses when necessary. In either event, as
sessions terminate and the addresses in the pool become available,
they can be allocated to translate new connections.
—Allows the 1-to-1, static translation of
a source IP address, but leaves the source port unchanged. A common
scenario for a static IP translation is an internal server that
must be available to the Internet.