--- premstud@vsnl.com wrote:

hi krishnan,

i think are confusing DSL with a leased line serial connection.

Actually, while I am not very familiar with DSL, I was told that it uses PPP over Ethernet. Now PPP is something I am fairly familiar with, and it is generally a point-to-point protocol (though of course you do have point-to-multipoint, but I think that would not apply here).

If this thing does run PPPOE the way I think it does (I may be wrong), you will find that you have a point to point link between the ISP and the user. The user gets a static IP address from the ISP's pool.

firstly for every DSL connection, if u started getting 4 addresses ( iam confused by what u mean by ur description of addresses here) then we would be out of stock of ip addresses in a day.

You're quite right, which is why the majority of home DSL subscribers do not get actual IPs - they sit behind a NAT or proxy, and get private IPs. However, when you ask the provider for a real IP address, he has to assign it out of a pool. For example, most ISPs in India start with a pool of 8192 IPs obtained from APNIC, of which they assign a chunk to dial-up customers, and use the rest for leased line customers etc. They can afford to assign real IP addresses to diaup customers because most dialup clients do not stay connected for very long, because of the telephone costs. However, a permanently on connection like Ethernet, cable modem, DSL etc. would be impossible to service this way - no ISP can afford to give these guys static IPs for free, so that they generally place them behind a NAT or proxy setup. The 24Online Ethernet home service used to use a Linux NAT ipchains setup - wonder if they're still using it.

I was actually imagining something like the way a leased line connection used to be provided, esp. in the bad old days when nobody understood things like unnumbered interfaces. Here what happend is that with a typical 64 KBPS leased line, the ISP gives anything between 8 to 16 static IP addresses, depending on how one negotiates with them. Assuming one has 8 IPs, the nature of subnetting means that one address gets to be the network address, and one gets to be the subnet broadcast address. Thus we would be left with 6 usable IPs. The ISP assigns a router port on his network for us to connect and route to the Internet. We would have to place a router to connect to his port and set it up so that the router will forward all packets destined for our subnet. Typically, the ISP assigns us a tiny /30 subnet (32 bits in an IP address - 30 = 2 bits, so that you get 2^2 = 4 addresses). Of these, again one becomes network address, one broadcast, and you're left with two addresses, of which one is assigned to the ISP's router and one to your router's outward port. It sounds weird, but believe me, I have seen more than one ISP do things this way!

This is a very simplistic solution, of course - the use of techniques such as unnumbered IP interfaces etc. would allow the serial link to run without an IP address, since it would work off the IP of the other router interfaces! Also, if the ISP allotted us a single address on one of his subnets for our external link, that too would work very well.

I mentioned the first scenario since it is the simplest way of doing things, and the most wasteful.

ur suggestion might be right , but i would like to modify it like this without ip forwarding,

           |--------| 
     ______|__     |--------|       

ISP | 1 | | | --------|WinBox | |LinuxBox| |__ ___2_| |________| | |----- To private network

Your system would work fine on an Ethernet, or any other topology that is not point-to-point. As I said, I am not very conversant with DSL. If the DSL link is point to point and needs to be deserialized, then it will not work. You will then need to route using the Windows box (ugh) and place the Linux box on the inner link.

It would probably work quite well with home Ethernet networks, however.

HTH,

Krishnan

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