IPv6 Addressing & Header Basics

Why 128 bits instead of 32

IPv4's 32 bits cap out at about 4.3 billion possible addresses, and between phones, laptops, IoT (Internet of Things) devices, and servers, that ran out long ago (NAT, or Network Address Translation, the topic later in this module, is largely the industry's workaround for that shortage). IPv6 uses 128 bits: roughly 340 undecillion addresses, enough that every device anyone will ever build can have its own globally unique address with room to spare.

How an IPv6 address is written

An address is written as eight groups of four hex digits separated by colons, like 2001:0db8:0000:0000:0000:ff00:0042:8329. Two shorthand rules make that readable:

• Leading zeros in each group can be dropped: 0db8 becomes db8.
• One consecutive run of all-zero groups can be compressed to ::, but only once per address (otherwise it would be ambiguous how many groups it stands for).

Compressed, that same address becomes 2001:db8::ff00:42:8329.

No more broadcast: Multicast does that job now

IPv6 has no broadcast address at all. Anything that used to rely on a broadcast in IPv4 (including ARP, or Address Resolution Protocol) is handled by multicast instead, sent to a specific group of interested listeners rather than flooded to everyone. Address resolution, the IPv6 equivalent of ARP, is a multicast-based protocol called NDP (Neighbor Discovery Protocol).