NATd question

Matt Edwards matthewedwards at
Sun Dec 7 23:15:31 PST 2003

There some marketing scams out there that some ISP use to get you to pay
just a bit more for service.  One common scam is as follows:

consumer:  "I have two computers.  I need to make sure they can both get on
the internet."  (Thinking: "I know my buddy did this with his setup")
ISP: "Oh you mean you need a second IP address, right?" (Thinking: "The poor
guy doesn't know he can do it with one and NAT server.  But I ain't telling
him that.")
consumer: "Will that let me get both of these connected?"
ISP: "Oh you bet.  It will only be $5($10/$15/maybe even $20) more dollars a
consumer: "Ok set it up.  Lets do that.  I really need to get these two on
the internet."



----- Original Message ----- 
From: "Vulpes Velox" <kitbsdlists at>
To: "Marwan Sultan" <Admin at>
Cc: "FreeBSD questions List" <freebsd-questions at>
Sent: Sunday, December 07, 2003 8:50 PM
Subject: Re: NATd question

> On Sun, 7 Dec 2003 19:56:14 +0300
> "Marwan Sultan" <Admin at> wrote:
> > Hello Everybody,
> >
> >
> >    Im on FreeBSD 4.8-R fresh installation.
> >    and this is the configuration i will use on my LAN
> >
> >    Internet ->connected-> DSL Router -> FreeBSD box -> HUB -> LAN
> >
> >    DSL Router has an IP and the real IP too.
> >
> >    FreeBSD box has 2 NIC's
> >    My question is:
> >
> >    Where and How to sign 2 IPs for 2 NICs?
> >
> >    which IP the NIC connected to DSL should sign?
> >    Which IP the NIC connected to HUB should sign?
> >    Which of these IPs will be as gateway IP the clients will sign?
> The DSL connection is probally going to be DHCP so just set the nic
connected to
> it for DHCP. Then for the lan select what ever private address you want.
> You will have to experiment a bit to figure out which is which.
> >    I have read the instructions of,
> >    and it says the gateway will be the BSD machine
> >    BUT this IP in my LAN is signed to the router automatilcaly.
> You can assign the gateway machine any private address you want. BTW if
you have
> a gateway why are you setting up a second?
> >    Can someone explain? advise? This machine will be only for this use.
> It basically works like this...
> LAN -> gateway -> natd -> dsl uplink device -> WAN
> Any thing on to a address not included in the subnet mask is going to
> throught the gateway. The usual subnet mask is so thus there
> 256 possabilities of address in the alloted space. Now lets say out
gateway is
> any thing not belonging to is going to leave
> the gateway. On it's way out that packet is going to be monkeyed, by natd,
> so it will know where to send the returned info. This is done so that it
> know which machine on the LAN it should be sent too.

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