Network Address Translation (NAT) provides the means for routers to manage communications between multiple computers on an internal network (Local Area Network - LAN), and computing resources on the Internet (Wide Area Network - WAN). Each computer on the LAN has its own identity (IP address) but externally a single identity (IP address) is used. The router maintains communication/connection information in a NAT table.
The NAT table is part of the NAT operation, which takes place within the router (or any NAT-enabled device). Each connection from an internal computer to the Internet is noted in the NAT table, the router's external IP address is substituted as the originating address then responses to that address from the external source are looked up in the table and internally routed back to the appropriate local computer.
A corollary of this process is that use of the NAT table also provides a simple form of security (firewall), since unsolicited traffic will not have a tracking entry in the NAT table the router can ignore the connection. Of course there are instances where unsolicited connections are expected, e.g. when gaming. A process known as Port Forwarding is used to cover this situation but further discussion of that topic is beyond the scope of this article.
So how does the NAT table work?
Each connection from the internal network (LAN) to the external network (WAN) results in an entry being made in the NAT table. Entries should expire after a short time and be deleted from the table but it appears that deleting entries is a less efficient process than creating them. In time this table can become congested such that browsing becomes slower. This comment also applies to other tables maintained by the router such as the DNS (Domain Name System) look-up table. That is why it is a good idea to reboot your router from time to time, particularly if you notice a gradual slowdown in throughput. Rebooting the router clears the entries in the tables and restores routing efficiency.
