Requires Fewer Registered IP Addresses. To connect to the Internet, a company must purchase IP addresses
from the American Registry for Internet Numbers (ARIN), which is the organization responsible for registering
and assigning IP addresses to those who wish to connect to the Internet. Currently, IP addresses are allocated
based on the size of the company that is requesting IP addresses. To prevent depletion of IP addresses on the
Internet, small and medium organizations receive fewer IP addresses, regardless of plans for future expansion. An
address hiding translator bypasses this limitation and ensures that you can continue to grow your network without
acquiring additional addresses. Because an address hiding translator distributes the control and allocation of valid
external IP addresses, it provides full connectivity and access to the Internet regardless of the size of your
network or the number of users that you support.
Use of Invalid Internal Addresses. Because many companies use invalid IP addresses within their intranets,
computers using those addresses cannot legally access the Internet. From the perspective of the routers, these
addresses appear to belong to a network that is different from the Internet. If you have used such addresses, you
may find that it is impractical to change them to valid internal addresses. The address hiding translator maintains
the integrity of your internal addressing schemes by mapping registered IP addresses to all internal addresses,
including invalid addresses.
Note: Invalid IP addresses are also referred to as reserved addresses, which are IP addresses restricted to
special purposes, such as internal domain or Internet service provider network usage.
How Dynamic NAT Works
For information to be routed correctly, each connected computer must have a globally unique transport address
that is identifiable by the routers that exist within the network of your Internet service provider, as well as those
routers that compose the Internet backbone. If the IP addresses are not unique, these routers cannot route network
packets. Those users who have duplicate IP addresses cannot be reached and cannot establish application
sessions.
Network Address Translation solves these problems by temporarily reassigning a registered IP address to an
internal computer that requests services across the Internet (or another external network). When residing on a PIX
Firewall unit, the address hiding translator acts as a buffer between the global Internet and the local IP networks
called subnets. The internal subnets only require IP addresses that are unique to that subnet level. When a
computer on one of these subnets sends traffic out over the Internet (thus traversing the PIX Firewall), the address
hiding translator strips the internal IP address (unique for that subnet) from the network packets and replaces that
address with a unique external address that is registered and assigned to that subnet or site.
Often, the address hiding translator contains a pool of external IP addresses, which enables more than one internal
computer to connect to the Internet at the same time. The pool contains those IP addresses that are registered with
the American Registry for Internet Numbers (ARIN),
http://www.arin.net
. When you allocate IP addresses for
your subnets, you must verify that those addresses do not conflict with the external IP addresses. Doing so
ensures that the external IP addresses remain unique, enabling the address hiding translator to distinguish among
computers. When a network packet is routed across the PIX Firewall, the address hiding translator replaces the
internal corporate address with a temporary external address. As soon as the application session is over (or the
idle time-out value is exceeded), the external address is returned to the pool, where it can be reassigned during a
new session request.
Summary of Contents for PIX 520 - PIX Firewall 520
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