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Answer Posted / zeeshan
Many organizations want the benefits of private addressing
while connecting to the Internet. Organizations create huge
LANs and WANs with private addressing and connect to the
Internet using Network Address Translation (NAT).
NAT translates internal private addresses into one or more
public addresses for routing onto the Internet. NAT changes
the private IP source address inside each packet to a
publicly registered IP address before sending it out onto
the Internet.
Using NAT on boundary routers improves security. Internal
private addresses translate to different public addresses
each time. This hides the actual address of hosts and
servers in the enterprise. Most routers that implement NAT
also block packets coming from outside the private network
unless they are a response to a request from an inside host.
NAT can be configured statically or dynamically.
Static NAT maps a single inside local address to a single
global, or public address. This mapping ensures that a
particular inside local address always associates with the
same public address. Static NAT ensures that outside
devices consistently reach an internal device. Examples
include Web and FTP servers accessible to the public.
Dynamic NAT uses an available pool of Internet public
addresses and assigns them to inside local addresses.
Dynamic NAT assigns the first available IP address in the
pool of public addresses to an inside device. That host
uses the assigned global IP address throughout the length
of the session. Once the session ends, the outside global
address returns to the pool for use by another host.
The address that one internal host uses to connect to
another internal host is the inside local address. The
public address assigned to the organization is called the
inside global address. The inside global address is
sometimes used as the address of the external interface of
the border router.
The NAT router manages the translations between the inside
local addresses and the inside global addresses by
maintaining a table that lists each address pair.
One of the more popular variations of dynamic NAT is known
as Port Address Translation (PAT), also referred to as NAT
Overload. PAT dynamically translates multiple inside local
addresses to a single public address.
When a source host sends a message to a destination host,
it uses an IP address and port number combination to keep
track of each individual conversation. In PAT, the gateway
router translates the local source address and port number
combination to a single global IP address and a unique port
number above 1024.
A table in the router contains a list of the internal IP
address and port number combinations that are translated to
the external address. Although each host translates into
the same global IP address, the port number associated with
the conversation is unique.
Since over 64,000 ports are available, a router is unlikely
to run out of addresses.
Both enterprise and home networks take advantage of PAT
functionality. PAT is built into integrated routers and is
enabled by default.
COMSATS-WAH
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