Peter said:
Not sure how the OP differs from my friends.
They already have the switch and your friend didn't.
Are there 2 different
types of ADSL modems out there? (Is this the NAT facility you
mentioned?)
Yes. Some have NAT and others don't. Depends on whether they were intended
to support one, and only one, computer or if they have provisions for more
than one, meaning NAT built in. Look up the specs for the modem and it
should say so.
Certainly the OP didn't mention what was being used to
connect with, but presumably it was connected directly to the switch in
order to try to allow more than one computer to make use of it.
They wanted all of them to work but only the first one that tried to, and
subsequently did, get a connection did work with 'connection' being the NIC
saying it's 'connected', which means it got an IP from the DHCP request.
They're just booting the machines (or enabling/disabling the NIC, or
plugging cables) and observing which one works: the first one up.
Not sure I quite understand why my friend's computer worked. I do know
that if you tried connecting the modem to a different computer then it
wouldn't work until you reset the modem (by completely powering it down)
because it was locked into the MAC address of the previous computer.
Yes, some do that. The ISP also checks the MAC at their end. It's how DHCP
keeps track of assigning IPs.
So, it really only wanted to allow 1 computer connected at a time which
makes me wonder where it was getting the MAC address from when going
through the switch. If it was fetching this from the first computer
that was connected when the network was set up, then I would have
thought that still only that computer would have been allowed to connect
to the internet. So how was it resolving the requirement for a MAC
address? And, if from one of the computers in the network, then how
were the other computers (with different MAC addressess which should
have caused the modem to refuse to allow traffic to/from I would have
thought) able to send/receive data?
Hope you can explain, or direct me to somewhere on the web that can do
so.
It's hard to explain what your friends system did when there's no
description of the equipment but a switch doesn't do NAT. Maybe you got a
router thinking it was a switch. Maybe it was a router with a switch, a
common situation, and that made you think it was a switch. Maybe you really
did get a switch and the modem had NAT. I can't tell without any model numbers.
What I can say, though, is if you have a typical DSL/Cable line that
provides only one IP then you have to have NAT somewhere, either in the
modem, standalone NAT router, or ICS in one of the computers, to connect
more than one computer to it and a switch won't work because it doesn't do NAT.
Consider: Each computer needs an IP and if the ISP provides only one then
where do the others come from? The NAT router. But switches don't have a
DHCP server and don't assign IPs... nor do they do NAT. They can't possibly
work unless there's NAT somewhere else, e.g. in the modem.
The other possibility is paying the ISP for more than one IP but since that
costs money I'd think one would know if they're paying for it.
(When I say NAT router I am speaking of the typical 'router' sold in
consumer stores that is actually a multifunction device including a router,
NAT, DHCP server and, usually, a firewall. A switch is just a switch.)