Assigning Dynamic IP Addresses to Callers

Dynamic addressing involves the assignment to each caller of the IP address that is defined in /etc/ppp/options.ttyname. Dynamic addressing occurs on a per-serial port basis. Each time a call arrives over a particular serial line, the caller is given the IP address that is defined in the /etc/ppp/options.ttyname file for the serial interface that is handling the call.

For example, suppose a dial-in server has four serial interfaces that provide dial-up service to incoming calls:

• For serial port term/a, create the file /etc/ppp/options.term.a with the following entry:

  :10.1.1.1

• For serial port term/b, create the file /etc/ppp/options.term.b with the following entry:

  :10.1.1.2

• For serial port term/c, create the file /etc/ppp/options.term.c with the following entry:

  :10.1.1.3

• For serial port term/d, create the file /etc/ppp/options.term.d with the following entry:

  :10.1.1.4

With this addressing scheme, an incoming call on serial interface /dev/term/c is given the IP address 10.1.1.3 for the duration of the call. After the first caller hangs up, a later call that comes in over serial interface /dev/term/c is also given the IP address 10.1.1.3.

The advantages of dynamic addressing include the following:

• You can track PPP network usage down to the serial port.

• You can assign a minimum number of IP addresses for PPP use.

• You can administer IP filtering in a more simplified fashion.