Understanding IPv6: A Quick Guide to Modern IP Addressing

IPv6 was developed to replace the exhausted IPv4 address space.

Although it has existed for nearly a decade, widespread deployment has been slow; adoption accelerated, reaching over 10 % of traffic in February 2016.

Modern computers and smartphones support both IPv4 and IPv6, so users may see either address.

IPv6 Addresses

IPv6 uses 128‑bit addresses written in hexadecimal, compared with 32‑bit decimal IPv4 addresses.

Each hexadecimal digit represents 4 bits, resulting in 32 hex digits grouped into eight 16‑bit blocks separated by colons.

Example IPv6 address:

Long IPv6 addresses can be shortened by omitting leading zeros, as shown in the example.

Network and Node Addresses

Like IPv4, IPv6 splits an address into a network portion and a node (interface) portion.

The address is divided into two 64‑bit halves: the high 64 bits for routing (network) and the low 64 bits for the interface, derived from the MAC address using the EUI‑64 format.

The high 64 bits themselves consist of a 48‑bit global routing prefix and a 16‑bit subnet identifier controlled by the network administrator.

Address Types and Ranges

Global Unicast – routable on the public Internet.

Unique Local – routable within a private network or VPN but not on the public Internet.

Link‑Local – not routable beyond the local link.

Global and Public Addresses

Global addresses are routable on the Internet and have been allocated since 2001. They correspond to IPv4 public addresses and are assigned to ISPs, which then allocate them to customers.

Internal Addresses – Link‑Local and Unique Local

IPv4 reserves private ranges such as 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, and 169.254.0.0/16.

IPv6 defines two internal address types:

Link‑Local – starts with fe80, confined to a single link and not routed.

Unique Local – starts with fc00::/8 (global) or fd00::/8 (local); used within an organization and not routable on the public Internet.

Link‑Local

Designed for communication within a local network; equivalent to IPv4's automatic private addressing (APIPA) 169.254.0.0/16.

Each IPv6 interface must have a link‑local address, which is self‑assigned without DHCP.

Unique Local

Intended for internal networks; routable inside the organization but not on the public Internet, analogous to IPv4 private ranges.

The address space is split into fc00::/8 for globally assigned unique local addresses and fd00::/8 for locally assigned ones; organizations typically use the fd00 prefix.

Using IPv6 Addresses in URLs

When an IPv4 address is used in a URL, the format is http://192.168.1.21/. Because IPv6 addresses contain colons, they must be enclosed in square brackets, e.g., http://[2001:db8::1]/.

IPv6 Loopback

The IPv6 loopback address is ::1. It can be pinged with ping ::1.