Where is the external IP address allocated from?

[#1]

In a separate thread Rest IP address @MisterW  explains why IP addresses can be quite sticky and a router needs to be offline for a long period before it is likely to be allocated a new IP address... 

@MisterW said: 

Dynamic IPs used to be allocated centrally , so if you disconnected for even a short time, there was a pretty good chance that while you were disconnected, someone else would connect and get your IP, so you got a new one when you reconnected.

Nowadays, IPs are allocated by the DLM from a local pool, so you need someone in that small pool to connect while you are disconnected. Obviously the chances of someone connecting in that pool are now reduced and you need to be disconncted for a much longer period to increase the chance

I didn't want to hijack the original thread but wondered a couple of things relating to this...

1) Does DLM run in the cabinet?  I had always just assumed it did as it seems to have access to the VDSL modem stats and line status....  if DLM does run in the cabinet is the IP address pool effectively at cabinet level.

2) What happens for FTTP, is the address pool managed at the "head end" exchange (whatever the head end is!)

Just curious - hope this is  the right place to ask this question

[#2]

@Ian06 I'm not entirely sure where physically the various bits of kit are located.

SIN 472 https://www.bt.com/bt-plc/assets/documents/sinet/sins/downloads/472v2p12.pdf gives a detailed explanation of how it all works and where the IP addresses are allocated (its not light reading though!)

AIUI IP address allocation for FTTP is the same as FTTC , they're both WBC. That would tend to indicate its at the headend exchange level rather than cabinet.

[#3]

Interesting read (well sort of!) - thank you.

What I took from the SIN document is that the IP address & DNS addresses are allocated by the RADIUS server when the PPPoE authentication happens... so when the CHAP messages are exchanged to authenticate the end user, if successful, the RADIUS server (either at BT premise (POP) or at WBC Customer network - eg Plusnet) will allocate an IP according to the agreement with the WBC Customer.   

So the IP address is either allocated from a pool of addresses in the BTW premise that the end user is connected (where BTW's RADIUS server is located) to or by the WBC Customer (Plusnet)... or that's how I read it.

Assuming that Plusnet have given pools of IPv4 addresses to BTW for each of the RADIUS servers in their POPs and BTW allocate an IP address locally from that pool when a Plusnet customer connects it would significantly reduce the pool size by geographic locations....  In the diagram from the SIN document I took it to mean each BTW POP (Point of Presence) would have RADIUS servers handling PPPoE authentication and handing out IP addresses:

By example the SIN document indicates BTW having 2 POPs in Birmingham and I assume each would have pools of IP addresses that it can allocate to their customers end user connections.

But then again I may have misinterpretted what I read...!!   Anyway thank you for helping answer the  question

[#4]

@Ian06 that's basically how I understand it. Exactly where the POPs are located , I'm not sure.

There is a slight difference in IP allocation between static & dynamic IPs.

For dynamic, they are allocated by the BT radius using the username & BBEU. For a static IP , its the bottom bit of the diagram in that an L2TP tunnel is created to the Plusnet BNGs, which then authenticate using the username & password, set up the PPPoE session and allocates the IP. associated with the account.