The Internet Protocol uses routing information to decide how to send datagrams to their destination. If your computer has more than one active IP interface or is connected to more than one IP gateway, the default routing information from the TCP/IP Control Panel may not be sufficient for IP to determine where datagrams should be sent.
The Routes window allows you to add or remove routes that IP uses to decide where to send datagrams. To define a route, you need to specify the range of destination addresses that should use this route, and where they should be sent to.
You specify where datagrams should be sent to by entering an IP Address for the "Gateway" field. This will generally correspond to the IP address of a host or gateway on a directly attached network that represents either a final destination or the next hop to which datagrams should be sent.
To specify the range of addresses that should use this route, you enter a template IP address for the "Destination" and indicate how target addresses should be matched against this template by selecting the Route Type as illustrated in the following examples:
Route Type = Network
IP addresses that match the network part of the "Destination" template are forwarded to this gateway. The network part of the IP addresses is determined by the network mask of the corresponding interface.
Route Type = Default Gateway, Destination = 0
All addresses that don't match another route should be forwarded to this gateway. If you specify more than one Default Gateway, datagrams will be split between them to balance the load on either gateway.
Route Type = Host
The address that matches the entire "Destination" will be sent to this host.
Route Type = Direct
The network part of the "Destination" must correspond to an IP interface on this machine. Addresses that match this network will be sent to the corresponding machine on the directly attached network (by using ARP to resolve the physical network address).
Route Type = Point-to-Point
The network part of the "Destination" will normally correspond to an IP interface on this machine. Addresses that match this network will be sent out the corresponding interface (address resolution is not needed since everything sent over this interface goes to the other side of the link).
Clicking on a row in the route table will automatically transfer the corresponding information to the data entry fields below the table. To remove a route, select the route from the table and press the "Remove" button.
You can save an IP configuration (interfaces and routes) to a settings document and then restore this configuration later by opening the corresponding document. Since the "Interfaces" and "Routes" window show the actual interfaces and routes the IP module knows about, you can only open one document at a time to restore a previous configuration (since there is only one IP module). Use the "Upon Open" popup menu in the Gateway window to control what happens when a configuration document is opened (configure only, configure and display). This is especially useful for placing a settings document in your Startup Items folder to configure multiple IP interfaces each time your Macintosh starts up.
Detailed instructions for using IPNetRouter are available from the Sustainable Softworks web site at <http://www.sustworks.com>.
