Understanding the network.pdf - Back to Home

The IOS NAT implementation can provide translation services for unregistered
inside addresses to registered outside addresses (the most common application of
NAT). Additionally, it can provide overlapping NAT translation. Overlapping
translation occurs when the inside address range is officially registered to another
publicly accessible network. NAT is also capable of providing round-robin TCP load
sharing across multiple hosts. With load sharing, a virtual host is created that is
announced as the legitimate address of the service. The NAT router serves as the
gateway to the virtual host, and as service requests come in, they are directed to
the "real" hosts that respond to the service request.
NOTE
In scenarios where PAT is used, in addition to inside/outside address translation
information, the transport layer (TCP/UDP) source and destination port information
are retained in the NAT translation table as well.
Inside Address Translation
Inside address translation is provided statically or dynamically. Static NAT mapping
translates an inside unregistered host address to an outside registered address.
Static mappings are used when an inside host needs to be accessed by external
hosts, as with an SMTP mail server. Static entries are created with the global
configuration command . Dynamic entries are inside-to-outside address translations made on a
temporary basis. A pool of outside addresses is allocated, and addresses are drawn
from the pool as translations are required. Dynamic inside-to-outside translations
remain until they expire (after 24 hours) or are cleared by an administrator. Here is
an example of a static translation entry:
sartre(config)#ip nat inside source static 192.168.4.36 12.14.116.5
The global configuration command is used to
create the outside NAT translation pool. The pool name is an identifier needed for
the global configuration command , which establishes dynamic NAT translation. The outside address range
should ideally be allocated within a classful or classless network range.