IPv6 (Internet Protocol Version 6)

Overview

IPv6 is the successor to IPv4 and was introduced to overcome the exhaustion of the 32-bit IPv4 address space. An IPv6 address is 128 bits long, which provides 2¹²⁸ (roughly 3.4 x 10³⁸) possible addresses.

The address is written as 8 groups of 16 bits (often called hextets or groups), each represented by 4 hexadecimal digits and separated by colons (:).

2001:0db8:0000:0000:0000:ff00:0042:8329

• 8 groups x 16 bits = 128 bits
• Hexadecimal digits are case-insensitive (ff00 equals FF00); but lowercase is the recommended canonical form

Address Structure

A typical IPv6 unicast address is split into two halves of 64 bits each:

Part	Length	Purpose
Network Prefix	64 bits	Identifies the network (routing prefix + subnet ID).
Interface ID	64 bits	Identifies the individual interface within that network.

The prefix length is written in CIDR notation, e.g. 2001:db8:abcd:1234::/64. A /64 is the standard size for a single subnet.

Shortening (Compression)

Two rules allow an IPv6 address to be written more compactly. They can be combined.

Rule 1: Remove Leading Zeros

Within each group, leading zeros may be omitted. At least one digit must remain per group.

2001:0db8:0000:0000:0000:ff00:0042:8329
2001:db8:0:0:0:ff00:42:8329

Rule 2: Collapse One Run of Zero Groups (::)

One single contiguous run of one or more all-zero groups may be replaced by a double colon ::.

2001:db8:0:0:0:ff00:42:8329
2001:db8::ff00:42:8329

Important: :: may appear only once in an address, otherwise the length would be ambiguous. If two equally long runs of zeros exist, the leftmost one should be compressed.

fe80:0:0:0:1:0:0:1   =>   fe80::1:0:0:1   (correct)
fe80:0:0:0:1:0:0:1   =>   fe80::1::1      (invalid, two "::")

Expanding

Expanding reverses the compression to recover the full 128-bit form. This is useful for comparison or manual subnet calculation.

1. Restore the :: – Count the groups that are present, then insert as many 0 groups as needed to reach 8 groups in total.
2. Pad each group – Add leading zeros until every group has 4 hexadecimal digits.

2001:db8::ff00:42:8329

Step 1 (restore zero groups, 5 groups present => insert 3):
2001:db8:0:0:0:ff00:42:8329

Step 2 (pad to 4 digits each):
2001:0db8:0000:0000:0000:ff00:0042:8329

Address Types

IPv6 has no broadcast. Its place is taken by multicast.

Type	Meaning
Unicast	One-to-one. Identifies a single interface; a packet is delivered to exactly that interface.
Multicast	One-to-many. Delivered to all interfaces that joined the multicast group.
Anycast	One-to-nearest. Shared by multiple interfaces; delivered to the topologically nearest one.

Scopes

The scope defines the region of the network in which an address is valid and routable. The two most relevant unicast scopes are:

• Link-Local – Valid only on the directly attached link (one segment). Not routed. Automatically configured on every IPv6 interface.
• Global – Globally unique and routable across the internet, comparable to a public IPv4 address.

Multicast addresses carry an explicit scope field (e.g. interface-local, link-local, site-local, global).

Zone Index for Link-Local

Because link-local addresses (fe80::/10) are not unique across multiple interfaces, the outgoing interface must be specified using a zone index appended with %.

ping fe80::1%eth0      # Linux (interface name)
ping fe80::1%12        # Windows (interface index)

Important Addresses

Address / Prefix	Name	Description
::/128	Unspecified	All zeros. Used as a source address before one is assigned (similar to 0.0.0.0).
::1/128	Loopback	The local host, equivalent to IPv4 127.0.0.1.
fe80::/10	Link-Local	Auto-configured, valid only on the local link, never routed.
fc00::/7	Unique Local (ULA)	Private addresses for internal use, not routed on the internet (similar to RFC 1918).
2000::/3	Global Unicast	Public, globally routable addresses.
ff00::/8	Multicast	All multicast addresses begin with ff.
ff02::1	All Nodes (link)	Multicast to every node on the link.
ff02::2	All Routers (link)	Multicast to every router on the link.
2001:db8::/32	Documentation	Reserved for examples and documentation, never used in production.

IPv6 vs. IPv4

Property	IPv4	IPv6
Address length	32 bits	128 bits
Notation	Decimal, dot-separated	Hexadecimal, colon-separated
Broadcast	Yes	No (replaced by multicast)
Loopback	127.0.0.1	::1
Auto-config	DHCP / APIPA	SLAAC + link-local