DI-308 ISDN Remote Router
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 | Local IP Addresses |
(for use with NAT) | (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.
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