Linux Networking question

Posted on 1997-09-23
Medium Priority
Last Modified: 2011-09-20
I would like to be able to have 4 computers at home with real ip's over a dialup to the internet.

server A is the ppp server on the internet.
server B is the ppp dialup on my local net.

I am not sure how to setup the netmasks or routing tables to correctly route the ip's of the machines connected to server B, or how to make server A route the additional ips to server B.  How does the Server A tell its router that it is a router also, or is that done by the netmask?  

From what I have been reading, machines connecting to server B must have a netmask of and the gateway ip of server B.  Is the gateway IP the IP of server B's modem or Netcard?  How do you setup the routing on server B so that packets get routed fromt the PPP interface to the ethernet interface?

I have found alot of how to's and faq's for Masquerading and Proxy, but no comprehensive routing ones.  If anyone could help me or point me to some more information I would greatly appreciate it.
Question by:skyman092397
  • 2

Expert Comment

ID: 1586633
OK here goes. Firstly you will require your isp to issue 4 ip addresses for you. Then you will need to assign one of these ip 's to each of the 4 computers as normal.

Let's say you had 32 ip's eg with a mask of ie This would enable you to assign an ip of to your dial-out server from home. Then assign any of the others to your other 3 machines.

What you would then need to do is to set a DEFAULTROUTE to via the interface ethernet0 for eg.

This means that say on one of the 3 other machines (not the dial-up machine) on the network there is a packet sent to a netwotk not part of your physical network. The defaultroute would tell the packet to go to the server On the server there is a defaultroute set to your isp and then your isp will deal with the routing from there.

This is a fairly simple setup and if you require very detailed instructions on how to do this you can e-mail me at jasons@aone.com.au,



Author Comment

ID: 1586634
I am sorry for being unclear about the fact that I am my own isp.

Server A is my machine at work on the internet
Server B is my machine at home

I am still unclear how the netmask and router(gateway???) affect the local and remote networking.  Additional information that may be of use:

Server A is Windows NT 4.0 Server with RAS
Server B is a Redhat linux 4.1 box and can dialup for a single ip


Accepted Solution

eckspurt earned 200 total points
ID: 1586635
There are a couple ways you can go.  I'll assume your work net is behind a firewall.  Then I'll assume you don't control the entire network at your office, and mention what you need to do in that case:

[Method 1]

Give Server_B the address  The 10.0.0 network is reserved for use within an organization.  

Set up routing on Server_A to reflect that will be on the PPP interface.  List it as the gateway for the 10.0.0 network, with a netmask of  Much simpler than dealing with tiny subnet masks, eh?  

Set up routing on Server_B to point at Server_A's IP address for its default route.  Give your ethernet interface the address, and assign a netmask.  Number all your other home systems within 10.0.0.  The hard part will be to set up IP Masquerading, but at least the HOWTO describes what you have to do with that to permit your other systems to talk to the Net.

[Method 2]

If you *do* have control of your office network, should I worry?  ;-)  That would remove the necessity for IP Masquerading, but you'd have to carve a chunk of addresses out of that network so you could assign them to your home systems, and you're back to worrying about tiny subnet mask.  You'd also have to change the netmask on every other system in your office if they're now using

So in summary, the answers to each of your original questions are:

Question 1 -
  (For method 1) Get the admin of your upstream router to add Server_A as the gateway for 10.0.0.  
  (For method 2) If your upstream router supports it (and trusts Server_A), use RIP to advertise routing for your home systems.

Question 2 -
  The gateway IP (at home) is that of Server_B's ethernet card.

Question 3 -
  Set Server_A as the gateway for the default route.  Then any packets destined for anywhere but 10.0.0 (in method 1) will be sent out the PPP interface.

With all the questions you asked or neglected to ask, I hope that's enough information to get you going!  Keep the NET-3-HOWTO handy while you're configuring things.


Author Comment

ID: 1586636
I never said anything about being behind a firewall and I am not.

If it wasn't evident about the netmask and the mentioning of the faq's, I DONT WANT TO MASQ or PROXY.

I am my own ISP by most peoples standards, I server out my own dialup lines.  Although I dont have control of the router.

All I want to do is route IP's though one dialup line that are on the same subnet.  I know its possible, I just don't know how.

btw I DIDNT accept this answer.

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