IPv6 CIDR Visual Explainer

An IPv6 address holds 128 bits written as eight hexadecimal groups, but it almost never appears in that full form: leading zeros are dropped and the longest run of zeroed groups becomes a double colon. The abbreviation helps reading and hinders comparing — two addresses written differently can be the same one.

Paste the address with its prefix and the page returns the expanded form, the canonical compressed form and what the prefix delimits. Expansion is the step that clears up most of the confusion: with all eight groups visible, you can compare addresses, check whether one falls inside a range and see where the prefix cuts.

On prefix sizes, the IPv6 convention differs sharply from IPv4. The standard block a provider hands a residential customer is a /56 or a /48, and each local network gets a /64 — which is the size stateless autoconfiguration expects. Subnets smaller than /64 break autoconfiguration, which is why economising addresses in IPv6 almost never makes sense: the space was sized so you would not have to.