How many bits are in an IPv6 address?

An IPv6 address contains 128 bits. That is four times the width of an IPv4 address, which contains 32 bits, and it gives IPv6 an address space of 2^128 possible values—approximately 340 undecillion addresses before accounting for reserved ranges and special uses.

The Internet Engineering Task Force formalized IPv6 in 1998 as the successor to IPv4. The larger address space was driven by the rapid growth of Internet-connected computers, phones, and other devices. IPv6 also supports hierarchical address allocation, expanded multicast, and features intended to make routing more scalable.

For readability, the 128 bits are written as eight groups of 16 bits. Each group uses four hexadecimal digits and groups are separated by colons, as in 2001:db8::1. Consecutive zero groups can be compressed with a double colon, but only once in an address. A common mix-up is assuming IPv6 has 64-bit addresses because many ordinary unicast networks divide an address into a 64-bit routing prefix and a 64-bit interface identifier; the complete address is still 128 bits.