when describing an IPv6 subnet, what does the /n notation mean, where n = 64, 128, etc.?

Question

I have seen IPv6 subnets described as /64 or /128, particularly by hosting companies. Does a /64 subnet mean there are 64 ipv6 addresses? Or 64bits worth of ipv6 addresses?

Answer 1

It means the number of bits in the network part of the address. If you compare with IPv4 there are 32 bits in an IP, of which any number can be assigned to the network part. So for example, if you have a /24, you have 8 bits left for the host part of the address (2^8=256).

If you extend this example to IPv6, which is 128 bits, for a /64 you would have 2^64= 1.8446744e+19 addresses available within the subnet. Also, a /64 is considered a 'normal' network in IPv6 terms (so end user LAN's would be /64's).