Broadband Subscriber IPv6 Assignment

IPv6 BNG Packet Flow Reference Guide RA (/64) vs DHCPv6 Prefix Delegation (/56) 

This document explains the end-to-end IPv6 packet flow in a Broadband Network Gateway (BNG), clearly separating the roles of Router Advertisement (RA) for the access link and DHCPv6 Prefix Delegation (PD) for subscriber network allocation. 

The flow is aligned with real-world ISP deployments using IPoE or PPPoE access models. 

1. Subscriber Session Creation When a subscriber connects (IPoE or PPPoE), the BNG creates a subscriber session containing IPv4 context, an IPv6 link (/64), and an IPv6 delegated prefix (/56).

 2. Router Solicitation (RS) The CPE sends an ICMPv6 Router Solicitation (RS) to ff02::2 (All Routers) asking for IPv6 router presence. 

3. Router Advertisement (RA) – /64 Link Prefix The BNG responds with an ICMPv6 Router Advertisement containing a /64 prefix. This prefix is used only for the WAN-side IPv6 address and Neighbor Discovery. It is equivalent to an IPv4 WAN address. 

4. WAN IPv6 Address Configuration Using SLAAC, the CPE configures its WAN IPv6 address from the advertised /64 and installs a default route towards the BNG. 

5. DHCPv6 Solicit – Prefix Delegation Request After WAN connectivity is established, the CPE sends a DHCPv6 SOLICIT message requesting an IA_PD, typically asking for a /56 prefix.

 6. DHCPv6 Advertise The BNG responds with a DHCPv6 ADVERTISE message, offering a delegated /56 prefix from the subscriber pool. 

7. DHCPv6 Request The CPE sends a DHCPv6 REQUEST to accept the offered delegated prefix. 

8. DHCPv6 Reply – Prefix Delegation Complete The BNG confirms the allocation with a DHCPv6 REPLY. The /56 prefix is now bound to the subscriber session and installed in the BNG routing table. 

9. CPE Internal Subnetting The CPE subdivides the /56 into multiple /64 LAN prefixes (home, guest, IoT, etc.) and sends RAs to internal hosts. 

10. Traffic Flow Outbound traffic flows from hosts to CPE to BNG to core. Inbound traffic is routed directly to the delegated prefix without NAT. 

11. Core Network View The core network only sees aggregated subscriber prefixes. Internal /64 LANs are hidden behind the CPE. 

12. Key Design Takeaways RA provides link plumbing (/64). DHCPv6-PD provides customer ownership (/56). IPv6 assigns networks, not just addresses, eliminating the need for NAT. 

Logical Flow Diagram (Textual) 

Internet 
 | 
 Core
 | 
 BNG
 |-- RA (/64 link) 
 |-- DHCPv6-PD (/56 site) 
 | 
 CPE 
 |-- LAN /64s
 | 
 Hosts 

IPv6 Routability and Commercial Model in Broadband Networks RA (/64) and DHCPv6 Prefix Delegation (/56) 

This reference guide explains whether IPv6 prefixes delivered via Router Advertisements (RA) and DHCPv6 Prefix Delegation (PD) are globally routable, and how IPv6 commercial models differ from IPv4 public IP pricing in broadband service provider networks. 

1. Are RA and DHCPv6-PD IPv6 Prefixes Routable? Yes. Both the RA /64 and the DHCPv6 delegated /56 are global unicast IPv6 prefixes and are technically routable on the Internet. The RA /64 is used only on the access link between the BNG and the CPE, while the /56 delegated via DHCPv6-PD represents the subscriber site network.

 2. RA /64 – Access Link Prefix The RA /64 provides WAN-side IPv6 addressing for the CPE using SLAAC. It is equivalent to an IPv4 public WAN address but is not intended for customer services or LAN use and is typically not visible beyond the access domain. 

3. DHCPv6-PD /56 – Subscriber Network Prefix The delegated /56 is the true customer IPv6 allocation. It is routed through the ISP core and advertised upstream in aggregated form. The customer can subdivide this prefix into multiple /64 LAN networks without NAT. 

4. Why IPv4 Public Addresses Cost Money IPv4 address space is exhausted. ISPs often need to purchase or lease IPv4 addresses from the secondary market, making public IPv4 addresses a scarce and monetized resource. 

5. Why IPv6 Prefixes Are Not Charged IPv6 address space is abundant. ISPs receive very large allocations from RIRs at minimal cost, making the marginal cost of assigning IPv6 prefixes effectively zero. Charging per IPv6 prefix would discourage adoption and is therefore avoided. 

6. Commercial Reality in ISPs Residential broadband services typically include IPv6 by default at no additional charge. Public IPv4 addresses or static IPv4 services may still incur additional cost, while business customers may receive larger IPv6 prefixes such as /48 as part of premium service tiers. 

7. Routable Does Not Mean Unprotected Even though IPv6 prefixes are globally routable, CPEs implement stateful firewalls by default. Security in IPv6 is provided by firewalls rather than NAT. 

8. Key Takeaway IPv4 assigns a paid public address due to scarcity, while IPv6 assigns a routable network as part of the standard service. ISPs monetize services and bandwidth, not IPv6 addresses. 

Analogy 

IPv4 public IP = Paid parking slot in a crowded city

IPv6 prefix = Free land with plenty of space