tart with a prefix, which defines which kind of address this is. For example, provider-based addresses have the prefix 010 and the fol- lowing structure: Starts with 010 to indicate that this is a provider-based address. The following 5 bits define the registry where the provider can be found. There are operating registers for North America, Europe, and Asia. The next 3 bytes define the provider number. A prefix is defined for local use addresses that have only local signifi- cance and each organization can use these addresses freely without con- flict. They are not propagated outside organizational boundaries, and suit well those who have isolated their network by firewalls from the global Internet [3]. If messages with local destination addresses are transmitted by accident to the public Internet, routers see that this packet does not belong there and discard it. Introduction to Telecommunications Network Engineering 6.6.4.6 IP Tunneling As we have seen, an IP address consists of network and host sections and each host accessible from the Internet must have the IP address of the network where it is located. There are several reasons why routing should sometimes be based on another address, not the original destination host address. When we want to use the public Internet for VPN connections we might want to hide host addresses in IP packets for security reasons. Another reason is to allow mobile terminals to roam to other networks and continue to use their home network address. Tunneling encapsulates an IP packet into a new IP packet, which has a new destination address of the cur- rent destination network to make roaming possible. Now routing is based on the outer IP header toward the network, where the mobile terminal is currently located. The original IP packet is de-encapsulated by a gateway in the destina- tion network, which knows the route to the foreign destination terminal. Tunneling is also used to improve the security of VPN connections. IP addresses of the gateways at the border of private and publi