Determining IP Addressing for Your PPP Link

To enable communications over the PPP link, the machine at one end of the link must know the host name and IP address of the peer host on the other end of the link. The PPP configurations often require a particular addressing scheme. This section explains the addressing schemes and where each should be used.

Specifying IP Addresses

On each endpoint machine, you specify addressing information in these places:

• /etc/asppp.cf configuration file

• /etc/inet/hosts file

• NIS+, NIS, or DNS databases, if applicable

When you edit the local machine's asppp.cf file, you must provide the host names and, in certain cases, the IP addresses for each endpoint machine to be on the link. For example, you must type either the IP addresses or host names for each endpoint as arguments in the ifconfig section in the configuration file:

ifconfig ipdptp0 plumb 192.99.44.01 192.99.44.02 up

See "Editing the Configuration File" for information regarding the format of /etc/asppp.cf.

Additionally, to enable communications, you must add the IP address and host name of the remote endpoints to the hosts database on the local endpoint by editing /etc/inet/hosts. This process is explained in "Configuring Network Clients".

Types of Addressing Schemes

You have a choice of several addressing schemes for PPP, depending on your configuration type. Before you edit the asppp.cf file and hosts database, you must decide on the appropriate addressing scheme for your configuration. These schemes include:

• Using the same IP addresses for the PPP endpoints as is assigned to their primary network interface in their local /etc/inet/hosts files

• Assigning a unique IP address for each PPP endpoint

• Assigning a new network number for the network created by the PPP link

Using the Same IP Address as the Primary Network Interface

This addressing scheme is appropriate for point-to-point links only. In this scheme, you specify the addresses of the primary network interface for each endpoint. (See Chapter 2, Network Services Overview for more information about the primary network interface.) These endpoints might be:

• Two standalone machines communicating over the PPP link (if they have existing IP addresses)

• Two network endpoints communicating over the PPP link

• Remote host connecting to a network dial-in server through a point-to-point link

• Dial-in server connecting to remote hosts through a dynamically allocated point-to-point link

When you edit the /etc/inet/hosts file on a local endpoint, supply the IP address of its primary network interface and host name and the IP address of the peer host on the other end of the link.

Creating a Unique IP Address and Host Name

In this method, you assign a unique host name and IP address to the PPP network interface. (You might want to call the interface hostname-ppp.) Use this addressing scheme for:

• Endpoint machines on a network used as a multipoint dial-in server.

• Machines on a virtual network.

• Remote host that uses a dedicated IP address for communicating with a dial-in server over a dynamically allocated PPP link. (Note that this is not a requirement for the dynamic link configuration.)

• Machine that is also configured as a router for a physical network, such as Ethernet or Token Ring.

• Machine in a standalone-to-standalone configuration that does not have an existing IP address. (The PPP interface becomes the primary network interface.)

You must specify the unique address and host name for the PPP network interface in the asppp.cf configuration file.

To create the new host name and IP address, add it to the /etc/inet/hosts file on the endpoint machines, as described in "hosts Database".

Assigning a Network Number to the PPP Link

You create a new network number for the PPP configuration when it involves:

• Virtual networks of computers communicating through PPP multipoint links (required)

• A multipoint dial-in server and its remote hosts (required)

• The PPP link between two networks, particularly when one or both of the network endpoint machines are also routers for a physical network (optional)

(See Chapter 5, Planning Your TCP/IP Network for information on network numbers.)

The PPP link becomes a virtual network, since it does not involve any physical network media. You need to type its network number in the networks database on all endpoint machines, along with the network numbers of the networks being linked.

The following sample shows an /etc/inet/networks file for an internetwork with PPP.

Example 22-1 /etc/inet/networks File for an Internetwork With PPP

kalahari      192.9.253
negev         192.9.201
nubian-ppp    192.29.15

In the sample file, kalahari and negev are two local area networks, and nubian-ppp is the name of the PPP link.