normally be set to the router’s link local address, although at this time, it varies by router
manufacturer.
The router, if configured as such, will tell the client to get its address via DHCPv6.
The
client will then ask the DHCP server for an address via a special local multi-cast
address (To simplify we will assume a local DHCP server).
The DHCP server will give
the client an address to use for a specified time range.
The client, if it needs to provide addresses to equipment connected to it on another
interface (such as is the case with a customer home router), may also ask for a prefix
delegation.
Prefix delegation refers to assigning a network to be used by a router on
the local subnet for connected devices on another subnet.
This is necessary as
Network Address Translation (NAT) is no longer available, or needed, in IPv6 and all
equipment that needs Internet access must have a globally routable (in IPv4 language a
“public”) address. Please note that most customer equipment (and much of the carrier
equipment) is not ready to deploy IPv6 at this time. As IPv6 fully matures over the
coming years more devices will employ these capabilities.
DHCPv6 is currently the only method in IPv6 for providing such a delegated network.
The current prevailing wisdom among network operators, regarding the size of the prefix
that should be assigned to each, is to assign a /48 prefix length per end-user. This may
sound like a lot. Indeed it is larger than the entire IPv4 address space which is 2
32
IP
addresses. A /48 is 2
80
IP addresses. However, it is designed to provide for future
expansion in the end-user’s network. Current standards also state that any subnet on
any network will be a /64.
And while all major computer operating systems have built-in support for IP addressing,
consumer equipment has been slow to adopt firmware that supports most, if any, of the
IPv6 constructs. Newly emerging consumer routers from manufacturers such as D-Link
are able to receive the prefix delegation and select a /64 for use internally. It is
expected that future consumer routers will have the capability, not only of maintaining
several discreet /64 subnets, but also of aggregating subnets to other equipment in the
household or business for various purposes. Each of these discreet internal subnets
will also require a /64 and a beginning subnet, received via Prefix Delegation, of
sufficient size so that the aggregation may occur.
In most cases an ISP will receive at least a /32 (a very small ISP - less than 4096
customers and less than 4 POPs). In a /32 there are 2
16
/48 subnets. An ISP can split
the /32 into 2
4
/36 subnets for use in their core network as well as in outlying pops.
Each subnet would be of size /64. Each POP with customers would also need a /40, for
example, for distribution of /48 subnets to customers. At most small ISPs this should be
an entirely reasonable allocation policy. This model also scales as medium and large
ISPs will receive appropriately larger allocations.
Chapter 9: DHCPv6 Configuration and Maintenance
58
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