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Iljitsch van Beijnum wrote : > So if an ISP has a /19, they use up /56 - /19 = /37 for the customers > in that /19. > If an ISP has a /19 and assigns /56s with 6rd to its customers, the computation is as follows: Let XXXX:e000::/19 be the ISP prefix. Let XXXX:e000::/24 be its chosen 6rd ISP prefix. Then, 6rd site prefixes are : XXXX:e0AA:AAAA:AA00::/56, where Y is "e" or "f", the first A is not e AAAAAAAA is the site v4 address Prefixes left for pure v6 sites are then: (a full /19 - (a full /24 - (a full /28))) In other words, *less than 1/32th* of the space has been used for 6rd. ( 1/32 = 1 / 2 ^ [24 - 19] ) With that practically negligible consumption, what has been bought is the capability to rapidly assign /56s to all v4 sites of ISPs . Note also that, once v4 addresses are no longer necessary for customer sites, this 1/32 of the space can be reused for pure v6 sites. Rémi |