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IPv6 Addressing

Because IPv4 only supports roughly 4.3 billion addresses, the rapid growth of the Internet led to an address shortage. IPv6 (Internet Protocol version 6) was developed to solve this problem permanently.

Structure of an IPv6 Address​

An IPv6 address is 128 bits long (compared to IPv4's 32 bits). This provides a virtually limitless number of addresses ($3.4 \times 10^38$).

IPv6 addresses are represented as eight groups of four hexadecimal digits, separated by colons.

Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334

Address Simplification Rules​

Because IPv6 addresses are long, there are rules to shorten them:

  1. Omit Leading Zeros: Within any group, you can remove leading zeros.
    • 0db8 becomes db8
    • 0000 becomes 0
  2. Double Colon ::: Consecutive groups of all zeros can be replaced with a double colon ::. This can only be done once per address.
    • The example above becomes: 2001:db8:85a3::8a2e:370:7334

IPv6 Subnetting​

IPv6 does not use traditional subnet masks (like 255.255.255.0). Instead, it relies entirely on CIDR notation (prefix length).

Example: 2001:db8:abcd:0012::/64

The /64 means the first 64 bits represent the network routing prefix and the subnet ID, while the remaining 64 bits are the interface ID (identifying the specific host).

A /64 subnet is the standard size for most IPv6 LANs and contains enough addresses for $1.8 \times 10^19$ devices!

Types of IPv6 Addresses​

Unlike IPv4, IPv6 does not use broadcast addresses. It uses three main types:

  1. Unicast: Identifies a single interface. Traffic sent here goes to one specific device.
    • Global Unicast: Equivalent to public IPv4 addresses (routable on the internet).
    • Link-Local: Equivalent to APIPA (169.254.x.x) in IPv4. Used only on a local network segment. Always starts with fe80::.
  2. Multicast: Sent to multiple interfaces in a defined group. Replaces IPv4 broadcasts. Starts with ff00::.
  3. Anycast: Assigned to multiple devices. Traffic is routed to the "nearest" device sharing that address. Great for load balancing and DNS.

Linux Configuration​

You can view your IPv6 addresses using the standard ip command in Linux:

ip -6 address show

You will almost always see a link-local (inet6 fe80:...) address even if your ISP does not yet support global IPv6.