Understanding the network.pdf - Back to Home

Network Address Translation
Internet access today (from both the home and office) is almost as commonplace as
the telephone. With the demand for access to the Internet today almost doubling in
size on a yearly basis, IPv4 address space is getting tight. Additionally, before
Internet access became widespread. The TCP/IP protocol was widely used on many
private enterprise networks. Since these networks were private (no Internet
access), many administrators used "illegal" IP address ranges. These "illegal"
address ranges, were IP addresses "assigned" to companies, by IANA for use on the
public Internet. Other network administrators use the IANA provided "unregistered"
IP address spaces to address their networks. Of course, in either instance, when
these networks then wanted to connect to the Internet they were forced into the
very unappealing and painful task of re-addressing the networks. Network Address
Translation (NAT) was primarily developed to address these troublesome issues.
NAT provides the capability to have a group of end-stations utilize an "illegal" or
"unregistered" IP address space. Sharing a group (or single address, if necessary)
of "registered" IP addresses to access hosts (that is, web, ftp, database servers and
so on). NAT, in its basic form, functions as described next.
An internal (privately addressed) host establishes a connection with an external
(publicly addressed) host. When the session is initially opened, the NAT gateway
router assigns an externally routable IP address to the internal host establishing the
external session (a one-to-one IP address translation). This address is then used to
readdress the source address of all the outgoing IP datagrams sent from the internal
host. As far as the external host is concerned, this external address is the real IP
address of the internal host. So, when the NAT gateway router receives traffic from
the external host with a destination address being used by an internal host, the
incoming IP packets are readdressed again. This time the IP datagram's destination
address is changed to reflect the actual internal address of the host. Along with
translation, it is the job of the router to keep track of which host is assigned which
address, how long they have had the address, and how long since the last time they
have used the address. After a defined period of inactivity, external addresses are
returned to the "address pool" and are reassigned to hosts who need them.
A variation of NAT known as Port Address Translation (PAT) is also often used to
provide addressing translation for the SOHO PPP dial connections. PAT allows a
single IP address to be used to manage multiple Layer 4 sessions (a one-to-many IP
address translation). PAT is often represented as NAT on equipment that typically
operates over PPP asynchronous links, where the IP address is assigned randomly to
the router each time the link is connected.