The primary difference between IPv4 and IPv6 addresses is length. IPv4 addresses are 32 bits long and IPv6 addresses are 128 bits long. This means that an IPv4 address is made up of 32 1s and 0s while an IPv6 address is made up of 128 of them – 128 binary digits. This massive length forces IPv6 addresses to be written using a different notation than IPv4 addresses and thus makes them very easy to distinguish from IPv4 addresses.

Introducing IPv6 | Understanding IPv6 Addresses is a post from don't panic – One Network Architect's View of Life, the Internet, and Everything.