I did almost understand it once - but I THINK the way it goes is that you open a default ATM circuit to BT's radius server which knows broadly 'who' you belong to, and that then opens a new circuit to the ISP's radius server using the credentials you have presented already..then you get logged in there and have a direct virtual circuit to their termination kit.
we were driving through Newmarket when my elderly mother asked what town it was. 'Edinburgh' I said. There was a pause. 'Its not how I imagined it would be' she said.
That's when we knew we had a problem.
at least 4 do.
WD40. Spawn of the devil.
I think it does. The circuits are delivered via ATM to the ISP. Not via Ethernet.
BT routes no packet at the IP level at all for 'customer ISPs' You can tell that with a traceroute.
Next hop is always somewhere in the ISPs 'virtual' machine room./
I see the Green Fool is still extant then...:-(
Thought that was Norfolk, not Suffolk? ;-)
Given the shape of the UK, to be able to get it 500 miles out should be difficult, but they managed for my static IP
In message , at
20:18:23 on Mon, 26 Aug 2013, Tim Hodgson remarked:Dynamic IP addresses are allocated from pools of numbers. In the old days it would have been a pool of numbers associated with a physical rack of modems, these days with cable and ADSL provision the pool might cover anything from a small area of a town to half the country.
So there are no useful generalisations to be made, especially when ISPs generally don't publish the physical reach of each pool of numbers (not to be confused with larger pools of numbers allocated by RIPE NCC etc).
In message , at 20:59:48 on Mon, 26 Aug 2013, John Williamson remarked:
It's a bit simpler than that - each modem had a static IP address, and the subscriber inherited that IP address for the duration of their session.
Some schemes existed to allow people to dial specific modems (rather than get a random one from a hunt group) which would mean they had a more consistent IP address.
And of course some ISPs went one stage further and routed their network such that subscribers were allocated a personalised static IP address. In those days IP addresses were in copious supply for any ISP who wanted to do that.
In message , at 20:56:03 on Mon, 26 Aug 2013, John Williamson remarked:
That sounds a bit like ADSL (although I'd doubt if the granularity was as low as one pool per exchange).
That sounds more like dial-up, although ADSL subscribers can also "disconnect". However, they would normally be given the same IP address when they reconnect (ie, turn their ADSL modem back on). That's the sort of address known as "fixed" (a dynamic address where you get the same one every time, more or less). Some ISPs also offer true Static IPs for ADSL (the same address for the lifetime of your contract).
But this is relatively simple scheme is being complicated by the use of carrier-grade NAT, where the subscribers are "behind" a relatively small number of public IP addresses.
No idea, to hide the underlying architecture from the service they sell I would think. You wouldn't want the ISP to have ATM to the customer as that would allow them to offer QOS.
Just because the link to the ISP is over ATM doesn't mean subscribers ATM circuits are presented at the ISP end. They could be delivered via ethernet
maybe originally. But that vanished when banks of modems were replaced by kit that plugged straight into BT supplied trunks, and radius servers came in.
In fact even when it was banks of modems connected to computers I cant recall there was necessarily a 'one per modem' IP allocation. PPP auth has generally been bound to the user, not the actual interface when stiing up IP addresses.
Except that now with 90% of people permanently connected everyone DOES essentially have an IP address - it just gets reallocated from time to time on dynamic setups.
And again, with the way backhaul is now run over ATM circuits, there is no need for any 'geographical' routing of IP packets to a given DSLAM. The ISP throws the IP packet into the ATM cloud and the routing to the DSLAM is done with ATM routing handled typically by BTs ATM network.
Its not that way for cable though, which is why cable customers have IP addresses that are related to the local cable hubs.
Its very similar to ADSL in that THESE days all the 'modems' are in fact banks of kit sitting at the end of an ATM backhaul for voice type circuits. I cant remember if the presentation is ISDN or not.
The difference with ADSL is that the DSLAM sits in the exchange. However the IP session only starts once the ATM conversation has been backhauled back to the ISP. So as far as the ISP is concerned the IP routing stops in their central machine room. IP is then tunnelled over te ATM to the dslam and then over DSL to the actual router/modem in the users premises.
So that from an IP perspective, the ADSL router/modem is DIRECTLY connected via and invisible virtual circuit to the ISPS edge ATM router in their machine room.
And that model is identical to modems. Expect that the DSLAM in the exchange is replaced by a different piece of kit in the ISPS machine room,. and voice circuits are routed over the BT bakhaul, instead.
But in neither case is there any need to know, or any knowledge of, which IP address is associated with which exchange.
That is the way things are done using BT to connect the customer to the ISP. All bets are off for cable. Certainly some cable companies - or the remains thereof all absorbed into Virgin I guess - used iP routing internally with IP pools allocated to distinct geographical areas.
ADSL subscriebrs are not normally given the same iP address. they will always be given a different one in general if they are on dynamic addresses. either on a round-robin or randaom allocation basis.
That is generally the case only on mobile comms.
The ATM does get to the ISP from the customer.
At some level BT provide that for them.
Indeed they could, But I dont think they are in fact.
not unless you are on 21CN arrangement.
so the concept of association of IP with a distinct geographical area simply does not exist.
In message , at 09:01:48 on Tue, 27 Aug
2013, The Natural Philosopher remarked:No, the banks of modems worked just the same, even if the modems became digital (and timeshared on one processor).
Nothing to do with that.
Yes it was.
That has nothing to do with it (unless it's an ISP who is going to great lengths to provide static IP addresses).
Unless they are behind carrier grade NAT.
It's still possible for the ISP to allocate the IP addresses based on regional pools of addresses at the NOC at his end of the ATM backhaul.
And then there's the ISPs who aren't BT Wholesale resellers.
Yes. But these aren't normally published.
In message , at 09:16:22 on Tue, 27 Aug
2013, The Natural Philosopher remarked:That's why I said "some".
And BT's retail product (for new subscribers).
John Rumm scribbled...
I'll have to work on my stalking skills.
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