Question on PPP extension, ZIPB and similar. (semi-bridge mode?)
 

Hello all,

ADSL access has only recently become kind of affordable in Greece, but despite the indisputable advantages it offers, I just can't live with the idea that no computer on the LAN has a real IP, and that all the masquerading and firewalling takes place in cheapo home router devices. Thus, I opted for an internal DSL modem which has proved greatly unreliable and extremely picky when it comes to distributions.

Only recently did I find out about a technology which enables a computer to bypass the routing features of the external "modem" and obtain a real WAN IP. This technology, two instances of which are named PPP extension mode (used by US Robotics) and ZIPB (used by some other manufacturers) work in the following way:
The DSL "modem" does the authentication with the ISP and provides an interface to the DSL network. Then, the aquired public IP address is assigned via DHCP to the eth0 (for example) interface connected to the modem, while routing and DNS servers' IP information is accordingly set the same way.
This is most commongly refered to as "semi bridge" mode.

My questions are:
1) Are there any disadvantages to using this solution compared to having an internal DSL controller, or USB DSL modem which provides a real ip to the computer anyway?

2) Why is the "semi-bridge" term used in this case? What way does this differ from a conventional bridged configuration?

3) Most home routers come with various performance limitations the most important of which being the number of simultaneous connections they can handle. I have always had the impression that this has to do with a sort of "NAT table capacity" or something. Whatever the case are these limitations negated by the aforementioned configuration? Does the fact that it is the computer which handles all the routing and masquerading functions help overcome these limitations?

4) Some routers (such as my USR 9106) have QoS functionality. In case the semi-bridge mode is used, does it make any difference whether this is enabled or not? Is QoS completely handled by the linux kernel in that case?

5) One solution to the real IP issue would be to use PPPoEthernet. Then it is the computer which initiates the internet connection and consequently gets its IP address from the ISP (I think). However, PPPoATM is prefered these days, since it has much lower overhead compared to PPPoE. In this case the connection is handled by the modem. The question is, what is this PPPoATM module in the kernel configuration about? Can it be used in order for the computer to handle the connection to the internet?

Thank you in advance for your time,
George