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Date:      Thu, 28 Nov 1996 09:30:52 -0100
From:      Darius Moos <moos@degnet.baynet.de>
To:        Jeff Newton <jeff@datapark.com>
Cc:        FreeBSD-questions <questions@freebsd.org>
Subject:   Re: Ifconfig (setup a point to point link)
Message-ID:  <329D69DC.7DBB@degnet.baynet.de>
References:  <329CCD73.7381@datapark.com>

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You can not span a continous C-class over two different network-
segments. That is what you are trying to do.
Your options:
 1. introducing a RFC-private net
 2. subnetting
 3. arping


  (Assuming the router is connected to the internet
   NIC1, NIC2 element of {ed0, ed1, ep0, de0, ...})

1.:
  ifconfig NIC1 inet xxx.xxx.xxx.1 netmask 0xffffff00
  ifconfig NIC2 inet 192.168.xxx.1 netmask 0xffffff00
  
  Disadvantage: You have to use proxying or network-address-translation

2.:
  ifconfig NIC1 inet xxx.xxx.xxx.253 netmask 0xffffff80
  ifconfig NIC2 inet xxx.xxx.xxx.1 netmask 0xffffff80

  Disadvantage: Waste of IP-adr.es
  Variants: You can tune the netmask to your needs:
      netmask 0xffffff80 gives you 2 subnets with each having 126 usable
      IP-adr.es plus broadcast and network.
      netmask 0xfffffffc gives you 64 subnets with each having 2 usable
      IP-adr.es plus broadcast and network.
      Tune the netmask to your needs.

3.:
  (LIP element of {xxx.xxx.xxx.1, ..., xxx.xxx.xxx.251};
   HWADRofNIC2 := ethernet-hardwareadr. of NIC2)
  ifconfig NIC1 inet xxx.xxx.xxx.253 netmask 0xfffffffc
  ifconfig NIC2 inet 192.168.xxx.1 netmask 0xffffff00
  arp -s LIP HWADRofNIC2         

  You have to arp for every possible LIP.
  
  Disadvantage: hardware-dependent to the HWADRofNIC2

Darius Moos.


Jeff Newton wrote:
> 
> Hi all,
> 
> After hours of banging my head against the wall trying to solve this
> problem, I thought I'd ask the gurus out there for some assistance.
> 
> I have a multi-homed freebsd box I'd like to use as a firewall/gateway.
> Here's the way I want to set it up:
> 
> router (XXX.XXX.XXX.254)
> |
> |
> point to point link
> |
> |
> NIC1 (XXX.XXX.XXX.1)
> gateway
> NIC2 (XXX.XXX.XXX.2)
> |
> |
> LAN
> 
> I've only got one C class and people have said you can't put the same
> network on two interfaces...others say that a point to point link should
> work (its easily done with Linux).
> 
> I figure ifconfig spits up at me cause I don't have the syntax righ for
> the point to point right.  Can someone show me how to do this?  Can this
> be done by adding static routes in a particular order.
> 
> Any advice would be much appreciated.
> 
> --
> Jeff Newton
> Systems Administrator
> Tantalus Communications
> Datapark Internet Services Inc.



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