Reading and Shortening IPv6 Addresses

An IPv6 address is 128 bits written as eight groups of four hexadecimal digits, separated by colons. Each group represents 16 bits, and eight times 16 is 128.

An example is 2001:0db8:0000:0000:0000:ff00:0042:8329. The colons replace the dots used in IPv4 notation.

IPv6 uses hexadecimal (base 16), with digits 0-9 and letters a-f. The letter a equals 10, b is 11, up to f which is 15.

Each hex digit represents 4 bits, so four hex digits make 16 bits — one group. Hexadecimal keeps the long 128-bit address compact and human-readable.

The first shortening rule removes leading zeros within each group. The group 0db8 becomes db8, and 0042 becomes 42.

Only leading zeros go; a group of all zeros (0000) shortens to a single 0. You cannot remove trailing or middle zeros, only those at the front of a group.

The second rule replaces one run of consecutive all-zero groups with a double colon (::). The :: stands in for as many zero groups as needed to total eight.

So 2001:db8:0:0:0:ff00:42:8329 becomes 2001:db8::ff00:42:8329, collapsing three zero groups into ::.

The :: may appear only once in an address. If it appeared twice, you could not tell how many zero groups each :: represents, making the address ambiguous.

When two separate zero runs exist, choose to collapse the longer one with :: and write the other run out as explicit 0 groups.

Start with 2001:0db8:0000:0000:0000:0000:0000:0001.

• Drop leading zeros: 2001:db8:0:0:0:0:0:1
• Collapse the zero run: 2001:db8::1

The shortened form is far easier to read and write while representing the exact same 128-bit value.

To expand, reverse the rules. For 2001:db8::1, count the groups present (2001, db8, 1 = three groups) and the :: fills the missing 5 groups with zeros.

Restore leading zeros too: it becomes 2001:0db8:0000:0000:0000:0000:0000:0001. Being able to expand helps when comparing addresses exactly.

The IPv6 loopback is ::1, the equivalent of IPv4’s 127.0.0.1. Fully written it is 0000:0000:0000:0000:0000:0000:0000:0001.

The unspecified address, all zeros, shortens to simply :: and means "no address," used before a device has been assigned one.

IPv6 uses CIDR-style prefix length too, written with a slash. The prefix is the count of network bits, just as in IPv4.

A very common prefix is /64, where the first 64 bits identify the network and the last 64 bits identify the host (the interface). Most LANs use a /64.

A typical IPv6 address splits into a 64-bit network prefix and a 64-bit interface identifier. The interface ID often derives from the device’s MAC address or is randomized for privacy.

This clean /64 split simplifies addressing and supports SLAAC, letting devices build their own addresses from the advertised prefix.

Reading IPv6 feels strange at first, but the two rules — drop leading zeros and collapse one zero run with :: — handle almost everything.

Practice shortening a few addresses by hand. Soon 2001:db8::1 will read as naturally as 192.168.1.1, and you will be ready to explore IPv6 address types.

Shorten the address.

You learned to read and shorten IPv6 addresses.

• IPv6 is 128 bits: eight groups of four hex digits, colon-separated.
• Rule 1: drop leading zeros in each group.
• Rule 2: collapse one run of zero groups with :: (only once).
• Loopback is ::1; unspecified is ::.
• A /64 prefix splits the address into network and interface halves.