Network Engineer Silver - IPv6 Subnetting

Unlike IPv4, you do not typically do subnetting of the bottom 64 bits of an IPv6 address. With very few exceptions, all IPv6 subnets are /64 (64 bit address prefix, 64 bit address suffix, or Interface Identifier). You cannot use smaller than a /64 if you use SLAAC, because it will generate address suffixes of 64 bits. You do not need to use anything larger than /64, because that is already larger than any conceivable subnet (it is 4.3 billion times larger than the entire IPv4 address space). So, all subnets are /64. Simple.

When we talk about IPv6 subnetting, we are referring to the first 64 bits of the IPv6 addresses, and usally only about the fourth group of 16 bits (the Subnet ID). If you only got a single /64 block, there is no subnetting to do. If you got a /48, you have 65,536 /64 blocks to manage. If you were allocated a block larger than a /64 and smaller than a /48 (e.g. a /60, /56, or /52), then you have some smaller number of subnets to manage. If you are with a giant organization that was allocated even more than a /48, you may have quite a few subnets to manage (more than 65,536).