The dial-up access server (a modem bank that provides PPP service to your clients) is configured to distribute an IP address to each client. There are a few ways to do that.
1. Assign an address to each modem port. Most access servers allow this configuration.
2. Dynamically assign IP addresses using DHCP from the access server by creating a pool of addresses that the access server can use.
3. Dynamically assign IP adresses using a DHCP server somewhere else on your network, and forwarding the address request from the client during connection to the access server.
To manage the addresses properly, the first solution is the easiest to track IP addresses, but it is also the most inflexible solution. That is usually the tradeoff with monitoring – we either choose ease of monitoring or ease of configuration.