Can't get IP by DHCP (Wired network)

[Patton Echols]

On 07/09/2007 02:36 PM, Dmitry Bond wrote:
> I have 1 external IP bought and a limit up to 3 internal IPs. When my
> ISP is giving me the IP (when I connect the desktop to the modem) it
> acts like that: "Hmm... Someone from MAC: xxxx (desktop mac)
> connected.... What do I have in my database??? Oh yeah, this man
> bougth an IP registered to this MAC. I'll give it to him... " and I
> get it. Maybe more complicated, but it's what is said by the admins on
> the local forums. And I've checked: my outer IP, but wrong MAC = no
> internet. Dunno how they do it, really... And for example, if I plug
> in a new device with MAC, that is unknown for my ISP, I should phone
> my ISP and register this MAC, and they enter it to some database.
> Otherwise, no internet  :(.
>
>   
Hmmm, not sure what you mean here. Do you have more than one 'puter 
attached to the modem? Are they switched? or through a router? If your 
problems are all related to one laptop and one wire to the modem, then 
I'm not sure what to do. Possibly the MAC is being misreported. Some 
other big heads need to work on it.

But you said "three internal IPs". If you have more than one computer 
attached (and just home networking, no servers, no DMZ) and they are 
just switched, then maybe you need a router. I'll get to how this solves 
your problem in a minute. First though, it is considered a "Bad Idea" to 
have a switched network exposed to the internet. In fact, there are 
plenty of folks who use a router to ""route" traffic to their one and 
only computer. Here is the configuration:

Computer 1---|
|---Router<--->Modem<--->ISP
Computer 2---|
Laptop ---|


What happens is that your ISP provides an IP via DHCP to the "computer" 
that it can see, the DHCP client in the router. The Router also contains 
a DHCP server that provides private addresses to the Computers in your 
network. Here is why this is a "Good Idea"

1. The Router can report to the ISP it's own MAC . . . OR, it can report 
the MAC of one of the computers on your network. As far as the ISP is 
concerned, the only traffic comes from that one and only source.
2. The Router does Network Address Translation (NAT) for the computers 
on the private side. This means that the IP of, say your laptop, is not 
routeable from the internet. That provides one layer of security, and 
that is a "Good Thing".
3. You can put as many computers in the private network as you want, 1?, 
2?, 10? . . . as far as the ISP is concerned, there is only one 
connection. And they can all connect at the same time . . .
4. The router will not care about the MAC of the laptop because you will 
tell it not to care.

Maybe I have misunderstood what the problem is, but I think this may be 
the way you want to go.

P.E.