System Administration Guide: Network Services

Creating an IP Addressing Scheme for Callers

Consider creating one or more IP addresses for all incoming calls instead of assigning a unique IP address to each remote user. Dedicated IP addresses are particularly important if the number of potential callers exceeds the number of serial ports and modems on the dial-in server. You can implement a number of different scenarios, depending on your site's needs. Moreover, the scenarios are not mutually exclusive.

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. When a call arrives over a serial line, the caller receives the IP address in the /etc/ppp/options.ttyname file for the call's serial interface.

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

With the previous addressing scheme, an incoming call on serial interface /dev/term/c is given the IP address 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

The advantages of dynamic addressing include the following:

Assigning Static IP Addresses to Callers

If your site implements PPP authentication, you can assign specific, static IP addresses to individual callers. In this scenario, every time a dial-out machine calls the dial-in server, the caller receives the same IP address.

You implement static addresses in either the pap-secrets or chap-secrets database. Here is a example of an /etc/ppp/pap-secrets file that defines static IP addresses.

joe    myserver  joepasswd
sally  myserver  sallypasswd
sue    myserver  suepasswd

joe, sally, and sue are the names of the authorized callers.


myserver indicates the name of the server.


joepasswd, sallypasswd, and suepasswd indicate the passwords for each caller.

IP Addresses and and are the IP addresses assigned to each caller.

Here is a example of an /etc/ppp/chap-secrets file that defines static IP addresses.

account1 myserver secret5748
account2 myserver secret91011

account1 and account2 indicate the names of the callers.


myserver indicates the name of the server for each caller.


secret5748 and secret91011 indicates the CHAP secret for each caller.

IP Addresses and are the IP addresses for each caller.

Assigning IP Addresses by sppp Unit Number

If you are using either PAP or CHAP authentication, you can assign IP addresses to callers by the sppp unit number. The following shows an example of this usage.

myclient ISP-server mypassword

The plus sign (+) indicates that the unit number is added to the IP address. Note the following: