Your client is considering acquiring a new IPv6 address block so that all Ethernet interfaces on the network receive addresses based on their burned-in hardware addresses, with support for 600…

A /70 subnet violates the EUI-64 requirement because it leaves only 58 bits for the interface identifier instead of the mandatory 64 bits, breaking SLAAC-based hardware address assignment.

A /58 prefix subneted into /64 networks yields only 2^(64-58) = 64 subnets, far fewer than the 600 VLANs required by the client.

A /68 subnet prefix length also violates EUI-64 by providing only 60 bits for the interface identifier, and a /60 block would produce only 16 /64 subnets - both issues make this choice invalid.

EUI-64 addressing requires the interface identifier to be exactly 64 bits, so each VLAN must be a /64 subnet; a /54 prefix provides 2^(64-54) = 1024 subnets, which is sufficient for 600 VLANs while correctly preserving the full 64-bit host portion needed for stateless address autoconfiguration (SLAAC) with burned-in hardware addresses.