3.2 KiB
Network Design
A first very general remark is that since IPv6 is a datagram protocol, whose routing relies on longest matching of address prefixes, the highest level of design decisions are identical to those for IPv4.
There is one constraint that does not apply to IPv6: there is effectively no theoretical limit to the number of hosts per subnet. (Mathematically, there is a limit of about 18.1018 nodes on a /64 subnet, but this is of no practical concern.) However, most network designers will never place hundreds or thousands of hosts on a single subnet, for performance reasons.
A network designer does, however, have more flexibility with IPv6. If an enterprise has a /48 prefix, 16 bits are available to identify more than 65 thousand individual subnets, a luxury for most IPv4 network designers.
Setting these details aside, there is no reason why an IPv6 network will have a different macroscopic design than an IPv4 network. The detailed approach will vary.
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If the intention is a "retrofit" where IPv6 support is added to an existing IPv4 network, major topology will not change, but items such as border routers, firewalls, interior routers, and DMZs will need to be upgraded accordingly. Clearly, a specific choice of IPv6/IPv4 coexistence mechanism must be made, and applied consistently. In the past, most networks have chosen the original dual-stack approach
[3. Dual stack scenarios](../3.%20Coexistence%20with%20Legacy%20IPv4/Dual%20stack%20scenarios.md)but designers should now also consider 3. Translation and IPv4 as a service. A priority will be adding comprehensive IPv6 support to the NOC and all its systems, before deployment to users. An equal priority will be training of all NOC and support personnel: they need to be IPv6 evangelists.
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If the intention is a "greenfield" deployment with no existing IPv4 network, the main topology will be conventional, but a specific choice of mechanism for IPv4 as a service must be made [3. Translation and IPv4 as a service]. The NOC must be designed from the start based on IPv6, with the ability to manage IPv4 as a service.
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A specific difference between a retrofit design and a greenfield design is that an existing IPv4 network almost inevitably has subnets limited to 256 or fewer hosts, often as few as 64. Since the normal subnet prefix in IPv6 is a /64, there is no such limitation in a greenfield deployment of IPv6. However, for practical reasons such as the rate of link-local multicasts, very large subnets should be avoided. As noted elswehere
[2. Address resolution](../2.%20IPv6%20Basic%20Technology/Address%20resolution.md)this applies particularly to wireless networks.
This chapter continues with a discussion of address planning, inevitably combined with subnet design.