2.5.1 General guidelines for addressing models

Addressing models for internetworks play a crucial role in improving efficiency, conserving bandwidth, ensuring scalability, improving general maintenance, and troubleshooting. When devising an addressing scheme there are several basic rules, as follows:

• Legal or private addressing Based on the size of your network and the extent of communications over public networks, decide whether you want to implement a registered or private addressing model. If using a registered set of class C addresses, ensure that you get a large enough block of contiguous addresses to allow for future expansion. Some ISPs may be short of addresses in contiguous blocks, and you may have to select another ISP. Private addressing is much more flexible, but you must control external interfaces rigorously and install NAT or application gateways in front of any public interfaces.

• Impose structure Before allocating addresses work out the overall structure of your network design. Focus on the main functional, topological, and geographical boundaries and work out where the key interfaces are. Your model should display a natural hierarchy that mirrors the inherent high-level structure and distribution characteristics of your organization. A structured model will be much more scalable than a flat or random model. If your network is large enough to warrant an ASN and meets the topology constraints imposed by IANA, use registered AS...