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DI-308 ISDN Remote Router
59
The actual information in the NAT table depends whether the router is implementing
NAT or NAPT.
NAT
This section discusses the NAT protocol as opposed to NAPT, which is discussed in
the next section.
NAT is the initial protocol set forth by RFC 1631 and provides a means in which
private networks can communicate with the Internet by using a small number of IP
addresses. In our discussion, we will use the example IP addresses listed in the table
below and the network diagram shown on page 58.
Global IP Addresses
(for use with NAT)
Local IP Addresses
(assigned to computers
on the local network)
200.100.50.1 192.168.100.2
200.100.50.2 192.168.100.3
200.100.50.3 192.168.100.4
200.100.50.4 192.168.100.5
200.100.50.5 192.168.100.6
192.168.100.7
192.168.100.8
192.168.100.9
192.168.100.10
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.