Here is how to put the 4200 into bridge mode.
»Bell Canada Internet FAQ
»How do I change into bridged mode on the 4200?
You just asked to have networking 101 explained in a single post. So many of the why's are going to be over simplified. The route of the problem is a thing called Double NAT, which will work in a lot of cases but in other's leave your network useless.
Why put it in your router? Because most routers are stuck in NAT mode. Meaning they translate packets off the internet to be routed the correct computer on the LAN. Actually the main purpose of the "router" hence it's name.
Now, most current DSL modems have a router built into them, but it's only has one output port, not the normal 4. So the simple way to set up is to put your name and pass in the modem using it's internal router and plug the modem into one of the LAN ports, leaving it's WAN port empty. Go in the router and turn off DHCP and it should be fine. Letting the modem's built in router to do the NAT.
A lot of the confusion is the way the router is integrated into the modem. Once you put your user and pass in most modern DSL modem/routers you are activating them to handle the PPPoE connection which in turn they then be default handle the NAT (nature of the beast).
The other way to not double NAT is put the 4200 in bridge mode above and put your user and pass in the router. This means the modem just handles the DSL and ATM/BRAS connection, but your router now handles the PPPoE connection, splitting the duties. Having the PPPoE in the router then lets it also handle NAT for your network (which nothing ahead of it trying to also NAT because the modem is now in bridge mode).
Does this cause more disconnects? Not unless the router has a problem. If you friends found putting user/pass in the router causing more disconnects, it's either coincidence or their router or it's firmware has a problem.
Now if their router is bad or flaky firmware, they don't have to buy a new one unless they need all four ports. Just tell them to plug the modem into a LAN port on the router, leaving the WAN port empty. Then go in the router web interface and turn off DHCP, or any PPPoE , basically turning the router into a wired/wireless switch.
So many of the commercial routers have so many gotcha's in them a lot of here have purchased routers that can be flashed with after market software, mostly x-wrt or tomato firmware. Then you have a very power stable Linux style router. Some go farther and set up an old computer with two NICs and run router software on top of Linux or something packaged like IPcop.--
| Speedstream 4200 Modem - 3m/384 plan | W98-W2KSP4-XPSP2 - All AMD | Buffalo WHR G54S with OpenWRT WR0.9 | 3 downstream switches feeding 6 total clients (no wireless) | Including the Data port on the side of my pork belly |