Gateways vs. Access Points

Some time ago, I had an occasion to set up a wireless network for some
friends.  They had a broadband connection to the Internet through a cable
modem provided by Mediacom and two desktop computers.  I suggested they buy
an 802.11b gateway and 802.11b cards for each computer they wanted connected.

After installing and connecting all the hardware they bought, each computer
seemed to be able to connect to the Internet fine, and we were able to do some
file and print sharing between them.  But it all wasn't fine.  I
quickly discovered that the computers could surf the net, but not at the same
time.  It is then that I discovered that they'd purchased an access point
and not a gateway.

A wireless access point does one thing.  It broadcasts a single RJ-45
jack to multiple wireless network cards.  It merely provides direct
connectivity to the public.  In this case, the public network sees all the
computers connected.  This is a problem because Mediacom (and many other
cable companies) will only assign you one IP address.  Not one for each
computer you have plugged into their network, just one period.  So when we
connected the AP and fired up the first computer, we were fine.  Mediacom
asked us to verify our new mac address, as expected.  Then we fired up the
second computer.  And again, we were asked to verify the mac address. 
Doing this, we got on the net with that machine too.  So all's well, right?

Wrong!  Being asked to verify a new mac address the second time was a
bad sign.  It meant Mediacom could see the mac addresses on each machine,
which told us that the wireless base was not fronting for us, but was exposing
each machine attached to it to the network, requiring each machine to obtain its
own IP address, which Mediacom will not do for more than one machine
simultaneously.

Ah hah!  It's an access point, not a router.

If my friends had purchased a wireless gateway instead of the simple wireless
access point, the gateway would have appeared to Mediacom as a single computer,
and only asking to update the mac address once, and that mac address would have
been for the gateway and not one for each of the computers.  With a
gateway, we not only insulate each of the internal computers from the Internet,
but we sort of fool Mediacom into thinking we only have one machine connected to
their network.  A gateway acts as the go-between for all Internet requests
coming from inside the network -- so instead of each computer requesting its own
IP address from Mediacom directly, each computer would ask the gateway (and not
Mediacom) for their IP address, which would in turn assign each machine a local
IP address from a bank of local subnet addresses.

Lesson learned.  Access points are fine if you're already behind a
firewall and are not limited to the number of IP addresses you can
request.  But if you're using a cable or DSL modem, make sure you use a
gateway to connect several computers to the network or you may have the same
problem we had.