IPv6 Part 2 -Unicast Address-

Apr 30, 2022 · 3 min read · IPv6 ·

Overview

IPv6 uses the following three types of addresses depending on the communication method.The broadcast address used in IPv4 is obsolete, and IPv6 uses multicast addresses. This article describes unicast addresses.

Type	Description
Unicast Address	An address used to communicate with a specific node.
Multicast Addresses	An address used to distribute specific data to multiple nodes. Multiple nodes/interfaces can hold a particular address.
Anycast Address	Multiple hosts or interfaces can hold a particular address. An address used to communicate with the nearest node among multiple nodes that hold the same address.

Previous articles can be found here.

IPv6 Part 1 -Address Representation-

Unicast Address

An address used to communicate with a specific node. There are two main address formats: Network Prefix and Interface ID.

Network Prefix
Indicates a location on the network in the same way as an IPv4 address; the Network Prefix length is usually 64 bits.
Interface ID
Like IPv4 addresses, it indicates a node in a subnetwork; Interface ID length is usually 64 bits.

Three types of unicast addresses are defined, starting with the following prefixes

Address	Type
2000::/3 [001]	Global Unicast Address(GUA)
fc00::/7 [1111 110]	Unique Local Unicast Address(ULA)
fe80::/10 [1111 1110 10]	Link Local Address

Interface ID

The Interface ID must be unique to identify the interface on the link. Multiple interfaces on a single node can use the same Interface ID. Interface IDs are determined in the following method.

Manual
Auto (EUI-64 or Random)

EUI-64

Generates a 64-bit Interface ID from a 48-bit MAC address.

Divide the MAC address by 24 bits and insert "FFFE" in between.
Invert the seventh bit (u-bit) from the top.

Random

The node itself automatically generates a 64-bit Interface ID using a random number. The automatically generated Interface ID is changed periodically to prevent the node from being identified.

Global Unicast Address (GUA)

Equivalent to an IPv4 global address; the Global Routing Prefix is assigned by the ISP, and the Subnet ID is determined by the organization.

Addresses used for special purposes

Addresses reserved for special uses. The following is a partial list.

Address	Description
2001:db8::/32	Addresses available only on the documentation
2002::/16	6to4 Address
2001:0000::/32	Teredo Address

Unique Local Unicast Address (ULA)

Equivalent to IPv4 private addresses: Prefix is fixed, Global ID is randomly generated by the organization, and Subnet ID is arbitrarily determined by the organization.

Prefix

The L-bit at the end of the Prefix is assigned as follows. 0・・・Defined in the future. 1・・・Locally Assigned. When ULA is used locally (=private), the L-bit is set to 1. Therefore, the current ULA is FD00::/8.

Global ID

Global IDs are generated using random numbers to avoid duplicate addresses when organizations merge.

Link Local Address

Interfaces using IPv6 addresses always have a Link Local Address; the Prefix is fixed (fe80::/10); all 54 bits after the Prefix are 0.