this is esp. useful for v6 hosts, where your provider might give you a changing prefix (we assume /64), but your interface ids stay the same.
but same mechanism also principally works for v4 (we assume /29), except that you usually do not get a v4 network where the network address is changing.
and when it comes to dynamic addresses, many people only get 1 ipv4 address anyway. the related host could then be used for aliaseses with ifid == 0 for all aliases.
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