6.4 Scaling Dial-on-Demand Routing
6.4.5 Network layer addressing
The remote routers and the central-site access routers have no need to connect to the Internet, so they use RFC 1597 addresses (NAT). The Class B address 172.16.0.0 is used for the entire access portion of the network, and Class C equivalent addresses are assigned to the remote routers. Each subnet gets one Class C equivalent (172.16.x.0 with a mask of 255.255.255.0), which makes addressing easy to manage. Network 172.16.1.0 is reserved for numbering the dialer cloud later if needed. (The dialer cloud is defined as the subnet to which all of the asynchronous interfaces are attached.)

Initially, the dialer cloud is unnumbered. If, in the future, the dialer cloud were to be numbered, the following questions must be considered:

  • Can the dialer cloud use the same subnet mask as the remote sites? If not, variable length subnet mask (VLSM) support will be required. (RIP version 1 does not support VLSM.)
  • Would the use of multiple subnetted Class C addresses cause discontiguous subnets at the remote sites? If so, discontiguous subnet support will be required. (RIP version 1does not support discontiguous subnets.)

In this network, these issues are not a problem. A mask of 255.255.255.0 can be used everywhere, so there are no VLSM concerns. All subnets are from the same major Class B network, so there are no discontiguous subnet concerns. The table in the graphic summarizes the addressing for the access portion of the network.

To facilitate an accurate routing table, all next-hop IP addressing must be accurate at all times. To accomplish this, the remote sites need to know the IP address that they will dial in to, and the central site needs to know the IP address of the remote site that has dialed in.

All central-site access routers use the same IP address on all of their asynchronous interfaces. This is accomplished by configuring the Dialer20 interface for IP unnumbered off of a loopback interface. The IP address of the loopback interface is the same on all of the central-site access routers. This way, the remote routers can be configured with the IP address of the router to which it connects, regardless of which router the remote router dials in to.

The remote router needs to announce its IP address to the central-site router when the remote router connects. This is accomplished by having the remote router start PPP on the central site using the EXEC command ppp 172.16.x.1. To support this, each central-site access router is configured with the async dynamic address interface configuration command.

Note: The autoselect feature allows the router to start an appropriate process, such as PPP, automatically when it receives a starting character from the router that has logged in. To use autoselect, a mechanism for supporting dynamic IP address assignment would be required, such as per-user address support in TACACS+.