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DI-206 ISDN Remote Router
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Please note that in the above table there are 9 users on the local network using 5
global IP addresses to access the Internet.
When a packet on the local network arrives at the router and needs to be sent to the
Internet, NAT will change the source IP address (for example 192.168.100.2) to a
global address (200.100.50.1, for example). If this packet generates a reply (as for
example, a request to view a web page will), NAT will change the destination IP
address on the reply packet back to the local IP address for delivery to the machine
on the local (stub) network.
The difference between static and dynamic NAT is that once the five global
addresses are manually assigned when using static NAT, they will never change. The
only way to change them is by using the console program to manually reassign them.
When using dynamic NAT, the router will map a local IP address to a global IP
address whenever a request is made. Since there are only 5 global IP addresses in the
example above, there can only be 5 mappings at any one time. In other words, much
like static NAT, only 5 local machines can access the Internet at any one time.
However, contrary to static NAT, the router will discard the mapping between the
global and local IP addresses after a certain length of time (which is quite long so
rarely happens), or after the session is finished (an example of a session is when
requesting a web page, the entire page has completed downloading). The most
common implementation of NAT is to define a range of dynamic addresses to be
used by hosts, but assign static addresses to your servers if you wish for them to be
accessible from outside your network.
Setting Local IP Addresses
When implementing NAT and thus creating a private network that is isolated from
the Internet, you can assign any IP addresses to host computers without problems.
However, the Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of IP Addresses specifically for private networks:
Class Beginning Address Ending Address
A 10.0.0.0 10.255.255.255
B 172.16.0.0 172.31.255.255
C 192.168.0.0 192.168.255.255
It is recommended that you choose local IP addresses for use with NAT from the
private network IP addresses in the above list. For more information on address
assignment, refer to RFC 1597,
Address Allocation for Private Internets
and RFC
1466,
Guidelines for Management of IP Address Space
.