Routing Tips for Multihoming with Cable and Dial-Up Networking

Introduction:

Win95 lets you use your Cable modem and Dial-Up Networking simultaneously, but if default settings are used, dialling in may cause existing cable TCP/IP sessions to be reset (ie. Telnet sessions killed, File transfers aborted). This document is intended t o explain how to take control of your machine and avoid this annoying situation.

Your computer has one address for the cable ethernet interface. ie. 207.134.108.12. When you dial-in, your computer gets a second address, say, 130.15.126.219. When you send information out to the net, it is sent in 'packets'. Packets sent from your comp uter are 'routed' to their destination. Normally all packets will be routed out through your cable ethernet card. When you dial-in, suddenly all these packets are forced out through the dial-in connection. Existing Telnet and FTP sessions will usually die when this happens, as they expect packets to keep coming from the same place (cable or dialin) after they are established.

So the idea here is to fix this 'routing' so that when you dial-in, packets keep sending from the cable interface. Of course we do want to send packets from the dial-up (that IS why we dialled in right? :). Because of the way routing works, we need to dec ide for ourselves what destinations should be accessed through cable and which destinations should be reached through dial-up. (If we aren't dialled-in, then we just resort to sending everything through the cable interface.)

Before I start in with the details, some people will need to check that they have the TCP/IP Protocol installed for their Dial Up Networking.

Adding the TCP/IP protocol for Dial-Up Networking:

You should have two separate TCP/IP Entries listed in your Networking Control Panel

For example:

TCP/IP -> Dial Up Adapter
TCP/IP -> D-Link 22X Adapter

If you don't have them both, (I assume you won't have the one for your Dialin Modem if so) then click on the 'ADD' button, select 'Protocol', Find 'Microsoft' in the list and choose 'TCP/IP'

Configure the TCP/IP properties for the new TCP/IP entry. You should be able to use whatever setup information that your Dialup ISP gives you on dialing in.

The only critical part is don't alter the settings on the 'DNS Settings' page, as ALL TCP/IP entries share this page, and if you change it then you will change the normal Cable settings. These normal Cable settings are best left alone, as it will not real ly affect your dialin session.

That may be all you require. If you find that stuff is going out the dialup that you prefer should go out the cable modem, or vice-versa, read on.

Dialup Examples:

The big trick is that before you dial in, you add specific routes for your current connections, so when you do dial in, those routes will be maintained. If this part gives you any trouble, try reading the Routing Basics section.

I use Cable and dialup all the time. I have to dial in to Queens to read the queens.* newsgroups. In order access both Queens and Cablenet's news servers simultaneously, I have to explicitly set the routes up myself.

Here are the routes before I dial in:

  Network Address          Netmask  Gateway Address        Interface  Metric
          0.0.0.0          0.0.0.0    207.134.108.1  207.134.108.12        1
        127.0.0.0        255.0.0.0        127.0.0.1        127.0.0.1       1
    207.134.108.0    255.255.255.0  207.134.108.12   207.134.108.12        1
  207.134.108.12   255.255.255.255        127.0.0.1        127.0.0.1       1
  207.134.108.255  255.255.255.255  207.134.108.12   207.134.108.12        1
        224.0.0.0        224.0.0.0  207.134.108.12   207.134.108.12        1
  255.255.255.255  255.255.255.255    207.134.108.12    207.134.108.12     1

Here they are after I dial in:


  Network Address          Netmask  Gateway Address        Interface  Metric
          0.0.0.0          0.0.0.0    207.134.108.1  207.134.108.12        2
          0.0.0.0          0.0.0.0   130.15.126.219   130.15.126.219       1
        127.0.0.0        255.0.0.0        127.0.0.1        127.0.0.1       1
       130.15.0.0      255.255.0.0   130.15.126.219   130.15.126.219       1
   130.15.126.219  255.255.255.255        127.0.0.1        127.0.0.1       1
    207.134.108.0    255.255.255.0  207.134.108.12   207.134.108.12        2
  207.134.108.12   255.255.255.255        127.0.0.1        127.0.0.1       1
  207.134.108.255  255.255.255.255  207.134.108.12   207.134.108.12        1
        224.0.0.0        224.0.0.0   130.15.126.219   130.15.126.219       1
        224.0.0.0        224.0.0.0  207.134.108.12   207.134.108.12        1
  255.255.255.255  255.255.255.255    207.134.108.12    207.134.108.12     1

Now I can't connect to the Cablenet News Server anymore.

Notice the 'metric' entry. Higher metrics have the effect of making a
route less favourable. As you can see, our beloved Cable is now
set to be less favourable. Regardless of the metric, we still have the
problem of which packets go where.

You can change the metric for a route. Here is an example:

route change 0.0.0.0 mask 0.0.0.0 207.134.108.1 METRIC 1

We don't care about the metric here since we're going to add 
specific routes.

I will add a route directing all packets destined for the range of Cablenet
addresses to be sent from the Cable interface.

C:\WINDOWS>route add 207.134.0.0 mask 255.255.0.0 207.134.108.1

C:\WINDOWS>route print

Active Routes:

  Network Address          Netmask  Gateway Address        Interface  Metric
          0.0.0.0          0.0.0.0    207.134.108.1    207.134.108.12      2
          0.0.0.0          0.0.0.0   130.15.126.219   130.15.126.219       1
        127.0.0.0        255.0.0.0        127.0.0.1        127.0.0.1       1
       130.15.0.0      255.255.0.0   130.15.126.219   130.15.126.219       1
   130.15.126.219  255.255.255.255        127.0.0.1        127.0.0.1       1
    207.134.108.0    255.255.255.0    207.134.108.12    207.134.108.12     2
    207.134.108.12  255.255.255.255        127.0.0.1        127.0.0.1      1
      207.134.0.0      255.255.0.0    207.134.108.1    207.134.108.12      1
  207.134.108.255  255.255.255.255    207.134.108.12    207.134.108.12     1
        224.0.0.0        224.0.0.0   130.15.126.219   130.15.126.219       1
        224.0.0.0        224.0.0.0    207.134.108.12    207.134.108.12     1
  255.255.255.255  255.255.255.255    207.134.108.12    207.134.108.12     1

yay! now I can access both the Queens news server and Cablenet news server.

Routing Basics:

When you send a packet, the computer first looks to see where the packet is addressed to. It then takes that address and looks it up in a table (the routing table). These table entries tell the computer which interface to send the packet from. Here's a si mple example:

Active Routes:

  Network Address          Netmask  Gateway Address        Interface  Metric
          0.0.0.0          0.0.0.0    207.134.108.1    207.134.108.12      1
        127.0.0.0        255.0.0.0        127.0.0.1        127.0.0.1       1
    207.134.108.0    255.255.255.0    207.134.108.12   207.134.108.12      1
    207.134.108.12 255.255.255.255        127.0.0.1        127.0.0.1       1
  207.134.108.255  255.255.255.255    207.134.108.12    207.134.108.12     1
        224.0.0.0        224.0.0.0    207.134.108.12    207.134.108.12     1
  255.255.255.255  255.255.255.255    207.134.108.12    207.134.108.12     1

To see what your computer's routing table looks like, open a DOS Window and type 'ROUTE PRINT'

What the Headings mean:

  Network Address          Netmask  Gateway Address        Interface  Metric

Network Address:

This is the packet destination address that the entry applies to. If the destination address matches this number, then this line will determine the interface it is sent from (cable or dialup). Since there is a gazillion addresses out there, we have a mech anism to specify a range of addresses. See the NETMASK for more info.

Netmask:

This value looks like an IP address, but it isn't. It's a number used to specify the exact range of addresses that the entry's Network Addresss applies to. A simple explanation is if it's a '255', then it means EXACT match, if it's '0' then anything will match. (numbers inbetween 0 and 255 specify a smaller matching subrange, don't worry about it). See the commented table for examples of how we use this.

Gateway Address:

Normally you can send packets addressed to from 207.134.108.1 thru 207.134.108.254 DIRECTLY from your ethernet card, ie. those computers are directly reachable from you. Termed your 'subnet'. To get packets to other places (like Microsoft) you send those packets to your gateway, and your gateway happily forwards the packets on your behalf.

Interface:

When a packet's address matches an entry, the entry's Interface is the device that the packet will be sent from.

Here is a line-by-line explanation:

  Network Address          Netmask  Gateway Address        Interface  Metric
          0.0.0.0          0.0.0.0    207.134.108.1    207.134.108.12      1

If no other entries match the address (ie. the general internet)
then send the packet to this gateway

        127.0.0.0        255.0.0.0        127.0.0.1        127.0.0.1       1

Send packets addressed to the loopback to the loopback :)

127.0.0.1 is the 'loopback' port, it's like a fake ethernet address. If
you connect to it, it just loops back to you.


    207.134.108.0    255.255.255.0    207.134.108.12   207.134.108.12      1

Send any packets addressed to 207.134.108.x (my subnet) directly from me

    207.134.108.12 255.255.255.255        127.0.0.1        127.0.0.1       1

Send any packets from me, destined for me, to the loopback adapter (makes
sense)

  207.134.108.255  255.255.255.255    207.134.108.12    207.134.108.12     1

Send any packets to the broadcast address (207.134.108.255) directly from me

This is a specially reserved address which sends packets that will be received
by every singe machine from 207.134.108.1 thru 207.34.108.254 Hence the term
'broadcast address'  -you broadcast to everyone in your little group

        224.0.0.0        224.0.0.0    207.134.108.12    207.134.108.12     1

The 'multicast address'. Packets addressed to 224.x.x.x will be sent from ALL
interfaces installed on my computer.

  255.255.255.255  255.255.255.255    207.134.108.12    207.134.108.12     1

Send all other broadcast packets directly from me. (don't worry about this one,
I can't explain it that well because I don't know :)

Troubleshooting:

Here is some basic tests you can do if you've been messing with your routing table and stuff stops working. Change a route, try ping tests, repeat. Don't be afraid to just sit there and frig with it. :) If desperate, go back to the Routing section and check that you have all the proper routing table entries.

ping 127.0.0.1
- See if you can reach yourself on the loopback port. If not, something major is wrong.
ping 207.134.108.111
- See if you can reach yourself at your own address. If not, is the Adapter installed in the Networking Control Panel? Did you delete the route accidentally?
ping 207.134.108.1
- See if you can reach your gateway. If not, check your wiring.
ping 207.134.42.11
- See if you can reach the Cable Server. If not, check your gateway address and your NETMASK
ping 207.68.137.59
- See if you can reach Microsoft. If not, check your default route.

Additional Info:

Seanster's Cable Modem Facts Page for Net Freaks!

Solutions for Multihoming and using Dial-Up networking in Win95

Linux NET-2/3-HOWTO

Created February 16, 1997

http://www.Seanster.com/Routing.html

//End of File