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Advanced Networking for Mactintosh Professionals


User Guide


Using an Unnumbered Interface

The "Unnumbered" check box may be used for point-to-point interfaces for which no unique local address is provided. If you have been assigned a block of IP addresses by your ISP for example, you would select "Unnumbered" to give your PPP interface an IP address from the same range assigned to your LAN (an "unnumbered" interface).

Let's do an example

Suppose your ISP has assigned you, a range of 16 addresses. Your network number is (prefix length 28) or your subnet mask is

Your gateway needs two IP interfaces:

  1. An Ethernet interface on your LAN
  2. A PPP interface connected to your ISP

For the Ethernet interface, we might use (mask For the PPP interface, we could use (mask

Because we are using an IP address from the same network range as our Ethernet LAN, we need to mark this as Unnumbered to tell Open Transport it is not unique. Normally in order to route between two IP interfaces they must have different network numbers yet we are assigning the same address to two different interfaces. Point-to-point interfaces are a special case because the far end of the link can already be identified so it doesn't have to have a unique IP address.

Next, since we specified our PPP address explicitly, we also need to specify a Default-Gateway (in the routes window).

Your ISP might have already told you a Default-Gateway or router to use, but you can also use a "pseudo-gateway" you make up yourself. The reason is that this gateway is just a place holder for the other side of the PPP link. Anything sent to this gateway is just sent to the other side of the PPP link. It doesn't matter if there is really a physical gateway with this IP address or not. [When you tell PPP to get its IP address from the server, IPNetRouter sets up a pseudo Default-Gateway for you automatically.]

According to the standard, a gateway must be in the same subnet as your network interface. This is why we made the subnet mask for our PPP interface (prefix length 24 instead of 28). We need to assign a gateway that is within this subnet, but not within your 16 address range (so IP can route to it).

A good choice would be [We don't want to use the all zero or all one's host number]. To enter this default route in the routes window, specify the Route Type as "Default-Gateway", leave the "Destination" field empty (to match any address), and enter in the "Gateway" field. Then press Add to update the actual Routes in the table above.

Finally, your ISP must configure their routers to route datagrams for your network range ( to your gateway machine (

This sets up IPNetRouter for conventional subnet routing without IP masquerading (Network Address Translation). Since you are not using NAT, this setup is transparent to all TCP and UDP traffic.

You can also use the "Unnumbered" feature with IP masquerading to give your PPP and Ethernet interface the same IP address.

Suppose you have a static IP address from your ISP (just one), and want to run WebStar on your gateway machine. Since WebStar uses the IP address in the TCP/IP control panel, if you use a private address for your Ethernet interface (like, hosts on the public Internet won't see it.

We can fool WebStar into using the correct IP address by setting the TCP/IP control panel to use our static IP address for the Ethernet interface. Next we use IPNR to setup our PPP interface with the same IP address and set "Unnumbered" to tell OT it isn't unique. Here's the tricky part. Since we don't have public IP addresses to assign to the other machines on our LAN, we'll create a second IP interface on our Ethernet with a private IP address ( as in the Getting Started examples.

IP will use this private IP interface to talk to our LAN, will use our PPP interface to talk to the public Internet, and we conveniently put our public IP address in the TCP/IP control panel where WebStar can find it.