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"
Let's do an example
Suppose your ISP has assigned you 22.214.171.124-126.96.36.199, a
range of 16 addresses. Your network number is 188.8.131.52/28 (prefix
length 28) or your subnet mask is 255.255.255.240
Your gateway needs two IP interfaces:
- An Ethernet interface on your LAN
- A PPP interface connected to your ISP
For the Ethernet interface, we might use 184.108.40.206/28 (mask
255.255.255.240). For the PPP interface, we could use 220.127.116.11/24
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 255.255.255.0 (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 18.104.22.168 [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 22.214.171.124 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 (126.96.36.199-188.8.131.52) to your gateway
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 192.168.0.1), 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 (192.168.0.1)
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.