Tenon Intersystems : PPP : >PPP.Routing.shtml

PPP Routing

Once a PPP link is established, routes must be established that carry traffic between the link and local MachTen application programs or across local network segments.

PPP Default Route

The simplest method of establishing a route to the PPP link is to use the defaultroute /usr/libexec/pppd parameter. See /etc/pppclient for information about specifying the defaultroute parameter. This parameter causes a pppd to make the system-wide default route from the newly established PPP link and it's associated internet addresses. If a link was established with a local internet address 128.0.0.1 and a remote internet address of 128.0.0.2 a route would be established to 128.0.0.2 using interface 128.0.0.1. This route would be made the system-wide default route which would be used in the event that no other route in the routing table matched the destination internet address.

The system routes can be displayed with the netstat command:

netstat -r

produces:

Routing tables Destination Gateway Flags Refs Use Interface INET routing: default 192.23.246.18 UG 0 0 ie0 127.0.0.1 127.0.0.1 UH 1 0 lo0 192.83.246 192.23.246.20 U 1 75 ie0

PPP Subnet Masks

In order to properly establish a route, many times, a subnet mask must be used to cause parts of an internet domain to be routed to an interface and other parts of an internet domain to be routed elsewhere. To support this feature, the /usr/libexec/pppd has a netmask parameter which is used in combination with any derived local internet addresses to segregate parts of a single internet domain into different logical subnet routes. The netmask parameter is specified by a netmask identifier followed by a numerical mask. This is an example of a netmask specification:

netmask 255.255.255.128

When a packet has a destination address that does not exactly match one of the local host addresses, the PPP link is examined as a possible destination for the packet. To determine if the PPP link qualifies as the "best" destination, the PPP link internet address is logically anded with the numerical subnetmask specified by the subnetmask parameter. The destination address from the packet is also logically anded with the subnetmask from the parameter. If the two logically anded results match, the PPP link is used to transport the packet a step closer to it's destination.