ewall by hiding internal IP addresses, and enables a company to use more internal IP addresses. Since the addresses are used internally only (inside the proxy/firewall) there is no possibility of conflict with IP addresses used by other companies and organizations. NAT is an official IETF standard, specified in RFC 1631. NAT has no doubt prolonged the life of IPv4. Tunnelling will be frequently used. Tunnelling can be used for private numbering schemes that are sent through publicly numbered tunnels. Another application type is tunnelling IPv6 numbers through an IPv4 core and a third type is tunnelling for protec- tion causes. For interconnect reasons there is a need for numbers that include operator identity. There is always a connection between the signalling plan and the numbering plan, for example between SIP and the numbering systems. SIP uses also e-mail-like addresses and telephony addresses, which correspond to IP numbers: John.doe@company.se 0123456789@company.se john.doe@176.24.106.26 The connection between routing and numbering can be illustrated by the large corporate voice networks of the VPN (virtual private network) type, often covering many countries. The first goal for the call set up is to find the SCP (service control point) housing the operator VPN software package. This requires agreements with transit operators, which must route the calls based on operator identity. The next phase is to find the receiving (B-) subscriber by means of a called party number. Understanding Changing Telecommunications - Building a Successful Telecom Business Numbering portability requires a flexible numbering register (FNR) for E.164 num- bering. The FNR enables number portability by providing subscribers with the ability to change network operator whilst retaining the original directory number. The FNR applica- tion reroutes messages to the correct node (e.g. HLR) or network (e.g. PDC or IMT-2000) where the subscription data is kept. 8.9.2 IP NUMBERING In a public IP network there are a lot of addresses to cope 