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computer has no knowledge of the internal local network. In fact, the 
local network is invisible to all computers outside of it, all information 
about it being stored in the router’s NAT table. And this NAT table can 
only be affected by computers from inside the local network. The router 
will only add new entries or mappings to the NAT table when it 
translates addresses on outbound packets. Thus, all traffic must 
originate from inside the local network. If the router receives a packet 
from the outside (from an intruder attempting to gain access to your 
network, for example), the router will examine the source address of 
the packet and look for a match in the NAT table in it’s attempt to 
deliver it to the correct local computer. Since no entry for this address 
exists in the NAT table, the router will drop this packet, denying the 
potential intruder of any access. 

If you wish, however, to give access to one of your computers to 
people on the Internet (your company’s web server, for example), then 
you must use a static NAT or NAPT assignment for them. When using 
static NAT, you would choose one of the global IP addresses at your 
disposal and map it directly to the local IP address of the web server. 
Thus, any packets coming from the Internet to that specific global IP 
address will always be routed to the web server. For static NAPT, you 
map specific global IP port numbers to the local IP address and port 
number. In both cases, the statically assigned IP address or port 
number is taken out of the pool that the router uses in the normal 
dynamic translation process, and the computer no longer benefits from 
the security provided by the address translation process. 

NAT 

This section discusses the NAT protocol as opposed to NAPT which is 
discussed in the next section.