What does it mean /64 in IPv6 [closed]

Question

I have vps with ipv6, such as

2605:6f00:XXX::XXXX:XXXX/64 

I don't understand meaning of /64. Does it mean that my vps has more than one ipv6 adress ?

Answer 1

The /64 is the prefix length. It is the number of bits in the address that is fixed. So a /64 indicates that the first 64 bits of the 128-bit IPv6 address are fixed. The remaining bits (64 in this case) are flexible, and you can use all of them. This means that when your ISP gives you a /64 they are giving you 2⁶⁴ addresses (that is 18,446,744,073,709,551,616 addresses).

There are several common prefix lengths for IPv6. A /64 is the size of a single LAN. As a customer you often get a /56 (256 LANs) or a /48 (65,536 LANs) from your ISP. You can then create multiple LANs which need a /64 each.

The exact number depends on the circumstances and the choices of the ISP. In a datacenter a single /64 is enough for a simple deployment with a single VLAN or a single server (possibly with virtualisation). Datacenter deployments with multiple LANs (for example the uplink VLAN of a firewall, a DMZ and one or more internet networks, or for a VPN concentrator) need more than a /64. Residential connections (DSL, FttH, etc) often get a /56 or a /48 so that the subscriber can have multiple LANs (home, office, guests, etc). Business connections always get a /48 so that they can use the available space to build a good addressing/numbering plan.

Edit: Looking again at your example I see that you explicitly specify the last 64 bits of the address. This might also mean that your ISP gave you one IPv6 address in a shared /64 (LAN) that the ISP operates.