Add advice on subnet size

5. Network Design/5. Network Design.md

@@ -41,6 +41,16 @@ approach will vary.
   The NOC must be designed from the start based on IPv6, with the
   ability to manage IPv4 as a service.
 
+- 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.