d, because the NAT server resides on the local network and must be used to initiate any con- nections to hosts on the other side of the router, the NAT server prevents some potential malicious activity coming from outside hosts. This improves the reliabil- ity of the local network, enhancing privacy by making it difficult for someone from outside the network to monitor individual usage patterns. Of course, this is less a function of NAT than a function of the rules that can be present in any router or firewall-blocking all inbound traffic unless that traffic is a reply to some previous outbound request. Internet Addressing and Address Resolution There are two main types of NAT: dynamic and static. In static NAT, the public IP address is always the same, allowing an internal host, such as a Web server, to have an unregistered private IP address and still be reached over the Internet. In dynamic NAT, a private IP address is mapped to a public IP address drawn from a pool of registered public IP addresses. By keeping the internal configuration of the private network hidden, dynamic NAT helps conceal the network from outside users. As always, it is important to also note any shortcomings. As my colleague, Dr. Art St. George of the University of New Mexico, notes, "While it is true that NAT serves many useful functions, it also serves to make end-to-end performance very difficult, if not impossible, to achieve and measure. On the LAN, use of NATs makes network diagnostics very difficult." IPv6 Addressing Because researchers predict that we may run out of IPv4 addresses as soon as 2009 ("IPv6: Friend or Foe," SearchNetworking.com, http://searchnetworking. techtarget.com/originalContent/0,289142,sid7_gcil156263,00.html?track=NL-81& ad=539879), a new version of IP has been developed, called IP version 6 (IPv6). IPv6 provides for an expanded address space. It uses 128-bit addressing (versus 32 bits in IPv4), which results in 340 undecillion unique addresses-that is, 340 billion billion billion billion-which is enough to s