One IP for multiple machines

[richb at pioneer.ci.net (Rich Braun)]

Mike Bilow wrote:
> This subject came up a few months ago.  I thought it was on this list...

I did a search of my archives, and didn't see any keywords related to what
I thought was the topic, but now that I know to search for "masquerade",
there was a discussion of the then-named Highway-1 service with IP
masquerading back in April.

> First, you should know that cable modems are usually configured as Dynamic Host
> Configuration Protocol (DHCP) agents, which means that they provide an IP
> address dynamically.

The service I'm considering claims to provide static IP addresses.  I use
DHCPd on my Linux box to assign addresses to the PC's, so converting over to
the 192.168.x.x addresses would be quite simple.

> There is DHCP client support available for Linux.

One of the headaches with using dynamic IP addresses would be trying to keep
one or more DNS entries pointing to your private system.  If you don't mind
putting your POP3 and web stuff somewhere else, I suppose you could get by
without a fixed address, but I prefer to keep private stuff private.
Naturally, ISPs recognize that many home users also feel this way, and set
their prices according to perceived value rather than cost of delivery.
(It does cost slightly more to hand out fixed IP addresses, since you have
to notify customers whenever you need to renumber, but in my experience you
wind up having to notify everyone anyway when a major network renumbering
needs to be done.)

> In the specific case of cable modems, my understanding is that the hardware
> (MAC) address of the Ethernet card is checked by the cable modem, probably to
> prevent you from using more than the one card authorized.  Of course, you can
> override the MAC address of an Ethernet card with "ifconfig" and fake it to
> anything you need, although this probably will not be necessary.

Good thing to ask about when subscribing to the service.  My guess is that
most suppliers wouldn't want to be too rigid about this, since customers
swap Ethernet cards all the time and the customer-service headaches of
reprogramming MAC addresses would get out of hand as customer base grows.

> Except for PPTP support discussed below, IP Masquerading is fully supported in
> the standard 2.0.30 kernel.

Ugh, I just checked my system.  It's running an antique kernel, vintage
almost 2 years...

> For the sake of completeness, I should say that may boxes which are
> fairly inexpensive will do the same kind of address translation as
> Linux.  Included in this class are products from Arescom, OpenRoute,
> and WebRamp, sometimes in the under-$400 price range.

That's good to know!  When I wear my professional-consultant or
friendly-neighbor hat, I often prefer to recommend stuff like this
rather than Linux hacks (which I wind up having to babysit forever).

Thanks to all who answered this query!  I'll find out how good the
service is.  RCN's coming to Somerville eventually but so far all
they've got is vaporware for Internet service.  So despite the three
major fiber nets promised here a couple of years ago (Time-Warner,
RCN, Nynex), my options today are ISDN from Bell Atlantic and the
various wireless services popping up.

Oh, and the brand name on the cable-modem box I saw at the demo is
General Instruments.  It's a misnomer since the service is microwave
rather than cable, but the specs are otherwise much the same as the
cable guys', except that the return path is 33.6 instead of 400K or
whatever.

-rich