Preparing an IPv6 Addressing Plan

A major part of the transition from IPv4 to IPv6 includes the development of an addressing plan. This task involves the following preparations:

• Obtaining a Site Prefix

• Creating the IPv6 Numbering Scheme

Obtaining a Site Prefix

Before you configure IPv6, you must obtain a site prefix. The site prefix is used to derive IPv6 addresses for all the nodes in your IPv6 implementation. For an introduction to site prefixes, refer to Prefixes in IPv6.

Any ISP that supports IPv6 can provide your organization with a 48-bit IPv6 site prefix. If your current ISP only supports IPv4, you can use another ISP for IPv6 support while retaining your current ISP for IPv4 support. In such an instance, you can use one of several workarounds. For more information, see Current ISP Does Not Support IPv6.

If your organization is an ISP, then you obtain site prefixes for your customers from the appropriate Internet registry. For more information, see the Internet Assigned Numbers Authority (IANA).

Creating the IPv6 Numbering Scheme

Unless your proposed IPv6 network is entirely new, use your existing IPv4 topology as the basis for the IPv6 numbering scheme.

Creating a Numbering Scheme for Subnets

Begin your numbering scheme by mapping your existing IPv4 subnets into equivalent IPv6 subnets. For example, consider the subnets illustrated in Figure 4-1. Subnets 1–4 use the RFC 1918 IPv4 private address designation for the first 16 bits of their addresses, in addition to the digits 1–4 to indicate the subnet. For illustrative purposes, assume that the IPv6 prefix 2001:db8:3c4d/48 has been assigned to the site. The following table shows how the private IPv4 prefixes map into IPv6 prefixes.

IPv4 Subnet Prefix	Equivalent IPv6 Subnet Prefix
192.168.1.0/24	2001:db8:fd3c4d:1::/64
192.168.2.0/24	2001:db8:3c4d:2::/64
192.168.3.0/24	2001:db8:3c4d:3::/64
192.168.4.0/24	2001:db8:3c4d:4::/64

Creating an IPv6 Addressing Plan for Nodes

For most hosts, stateless autoconfiguration of IPv6 addresses for their interfaces is an appropriate, time saving strategy. When the host receives the site prefix from the nearest router, Neighbor Discovery automatically generates IPv6 addresses for each interface on the host.

Servers need to have stable IPv6 addresses. If you do not manually configure a server's IPv6 addresses, a new IPv6 address is autoconfigured whenever a NIC card is replaced on the server. Keep the following tips in mind when you create addresses for servers:

• Give servers meaningful and stable interface IDs. One strategy is to use a sequential numbering scheme for interface IDs. For example, the internal interface of the LDAP server in Figure 4-1 might become 2001:db8:3c4d:2::2.

• Alternatively, if you do not regularly renumber your IPv4 network, consider using the existing IPv4 addresses of the routers and servers as their interface IDs. In Figure 4-1, suppose Router 1's interface to the DMZ has the IPv4 address 123.456.789.111. You can convert the IPv4 address to hexadecimal and use the result as the interface ID. The new interface ID would be ::7bc8:156F.

  Only use this approach if you own the registered IPv4 address, rather than having obtained the address from an ISP. If you use an IPv4 address that was given to you by an ISP, you create a dependency that would create problems if you change ISPs.

Due to the limited number of IPv4 addresses, in the past a network designer had to consider where to use global, registered addresses and private, RFC 1918 addresses. However, the notion of global and private IPv4 addresses does not apply to IPv6 addresses. You can use global unicast addresses, which include the site prefix, on all links of the network, including the public DMZ.