Ed Horley wrote another great post arguing you don’t need Unique Local Addresses in an IPv6 network … and I couldn’t figure out what the problem was until I got the underlying context: it seems many engineers try to transplant their IPv4 mentality into IPv6 world and see ULAs as a nice replacement for RFC1918 with NAT66 or NPT66 on the private network edge. No wonder Ed argues against that.
While we still cannot escape the throes of NAT66/NPT66, you shouldn’t use ULAs as 1:1 RFC1918 replacement. Remember that every IPv6 host has more than one IPv6 address per interface. One of those addresses might be an ULA address (for intra-company communication), another one a public (PI or PA) IPv6 address (for global connectivity).
Using ULAs in combination with public IPv6 addresses is identical to the solutions Ed proposed in his blog post (see Figure 5 in his post), with a crucial difference: Ed’s solution requires you to modify IPv6 source address selection prefix policy entries, while ULA+global combination (usually) works with the default settings of most operating systems.
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