 field that prevents this from happening. The time-to-live (TTL) field is initialised by the sender to a certain value, usually 64, and reduced by one each time the packet passes through a router. When TTL get down to zero, the packet is discarded and the sender is notified using an Internet Control Message Protocol (ICMP) "time-out" message. Building Router Tables The clever part of a router's job is building its routing table. For simple networks a static table loaded from a start-up file is adequate but, more generally, Dynamic Routing enables tables to be built up by routers sending and receiving broadcast messages. These can be either ICMP Router Solicitation and Router Advertisement messages which allow neighbouring routers to ask "Who's there?" and respond "I'm here", or more useful RIP (Router Information Protocol) messages, in which a router periodically broadcasts its complete router table onto the network. Other RIP and ICMP messages allow routers to discover the shortest path to an address, to update their tables if another router spots an inefficient routing and to periodically update routes in response to network availability and traffic conditions. Chapter Two A major routing challenge occurs in mesh or mobile ad-hoc networks (MANETs), where the network topology may be continuously changing. One approach to routing in MANETs, inspired by ant behaviour, is described in the Section "Wireless Mesh Network Routing, p. 345". Network Address Translation As described in the Section "Private IP Address, p. 15", RFC 1918 defined three sets of private IP addresses for use within networks that do not require Internet connectivity. However, with the proliferation of the Internet and the growing need for computers in these previously private networks to go online, the limitation of this solution to conserving IP addresses soon became apparent. How could a computer with a private IP address ever get a response from the Internet, when its IP address would not be recognised by any router out in the Internet as a v