IPv6 Fundamentals: What It Is and Why It Matters

Why IPv6 Exists#

IPv4 addresses ran out. Not "might run out" or "could run out"—they actually did. IANA allocated the last blocks to regional registries in 2011. By 2015, most regions had exhausted their pools.

IPv4's 32-bit address space gives us about 4.3 billion addresses. That seemed infinite in 1981. But smartphones, IoT devices, servers, and everything else connected to the Internet consumed those addresses faster than anyone predicted. We worked around it with NAT, carrier-grade NAT, and increasingly complex hacks. Those hacks work, but they add latency, break end-to-end connectivity, and make network administration harder.

IPv6 uses 128-bit addresses. That's 340 undecillion addresses—enough to give every square meter of Earth's surface billions of addresses. We won't run out.

The 128-Bit Address Format#

An IPv6 address looks like this:

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

Eight groups of four hexadecimal digits, separated by colons. Each group represents 16 bits (4 hex digits × 4 bits per digit = 16 bits). Eight groups × 16 bits = 128 bits total.

Hexadecimal uses 0-9 and a-f. Case doesn't matter—2001:DB8::1 and 2001:db8::1 are identical.

Compression Rules#

Writing out all 32 hex digits gets tedious. IPv6 has two compression rules to shorten addresses.

Rule 1: Drop Leading Zeros#

You can remove leading zeros from any group:

2001:0db8:0000:0042:0000:8a2e:0370:7334
     ↓
2001:db8:0:42:0:8a2e:370:7334

Each group still needs at least one digit. You can't compress 0000 to nothing—that's what rule 2 handles.

Rule 2: Replace Consecutive Zeros with ::#

One sequence of consecutive all-zero groups can become ::. This only works once per address. Using it twice would be ambiguous—you couldn't tell how many zero groups each :: represents.

2001:db8:0:0:0:0:0:1
        ↓
2001:db8::1

Pick the longest run of zeros. If you have two equal runs, compress the first one (though most tools compress the leftmost by convention).

Practical Examples#

Here's how compression works on real addresses:

Try different formats with our IPv6 Validator to see how compression works.

IPv6 vs IPv4: More Than Just Bigger#

Most comparisons focus on address space. That's fair—it's the main reason IPv6 exists. But the protocol redesign fixed other issues too.

Key Improvements Beyond Address Space#

Simplified Header#

IPv4's header has 12 required fields and options that extend it to 60 bytes. Routers have to parse all of it for every packet. IPv6's header is always 40 bytes with 8 fields. Extension headers handle anything extra, but most packets don't need them. Routers process packets faster.

No NAT Required#

With IPv4, almost every home and office network hides behind NAT. NAT breaks protocols that embed IP addresses (like SIP for VoIP), makes peer-to-peer connections harder, and adds state to routers. With enough IPv6 addresses for everything, devices can have globally routable addresses. Your laptop, phone, and smart fridge can all be reachable end-to-end if you want them to be. (Firewalls still exist—you can block inbound traffic without NAT.)

Auto-Configuration (SLAAC)#

IPv6 devices can configure themselves using SLAAC (Stateless Address Autoconfiguration). The router advertises the network prefix, and devices generate their own addresses. No DHCP server required. You can still use DHCPv6 for centralized management, but basic connectivity works without it.

IPsec Built-In#

IPv4 added IPsec years after deployment. IPv6 included it from the start. The design is cleaner. (IPsec is now recommended rather than mandatory, but the integration is still better than IPv4's retrofit.)

Special Addresses You'll See#

Loopback: ::1#

The IPv6 loopback address. Same purpose as 127.0.0.1 in IPv4—traffic to this address stays on the local machine. IPv6 only has one loopback address instead of IPv4's entire 127.0.0.0/8 block.

Unspecified: ::#

Represents "no address" or "any address." You'll see this when a device doesn't have an address yet, or in listening socket configurations to bind to all interfaces.

Link-Local: fe80::/10#

Every IPv6 interface automatically generates a link-local address starting with fe80::. These addresses only work on the local network segment—routers won't forward them. They're essential for neighbor discovery, router advertisements, and local communication. Your interface might be fe80::1 or fe80::a4b2:c3d4:e5f6:7890, depending on how the suffix is generated.

Documentation: 2001:db8::/32#

Reserved for examples and documentation. Any address starting with 2001:db8: is guaranteed to never be assigned in the real Internet. That's why you see it in tutorials, RFCs, and these examples. If you're writing documentation, use 2001:db8:: prefixes so readers don't accidentally configure production addresses.

IPv6 in URLs#

IPv6 addresses in URLs need square brackets. Without them, the colons in the address would conflict with the port separator:

http://[2001:db8::1]/
http://[2001:db8::1]:8080/
https://[2001:db8:85a3::8a2e:370:7334]/api/endpoint

The brackets make it clear where the address ends and the port begins. You'll use this notation in browser URLs, configuration files, and anywhere else that combines an IPv6 address with other components.

Where We Are Today#

IPv6 adoption is real but uneven. Google reports about 40% of their traffic arrives over IPv6. Major cloud providers (AWS, Google Cloud, Azure), CDNs (Cloudflare, Akamai), and ISPs (Comcast, T-Mobile, Deutsche Telekom) have strong IPv6 support. Mobile networks often run IPv6-only with translation back to IPv4 when needed.

Corporate networks lag behind. Lots of internal tools and security appliances still treat IPv6 as optional. That's changing, but slowly.

The good news: IPv4 and IPv6 run side-by-side. Dual-stack networks support both. You don't have to flip a switch and convert everything overnight. But if you're building new systems, design them for IPv6 from the start. Adding it later is harder.

Related Articles#

• IPv6 Address Types - Learn about unicast, multicast, anycast, and special address ranges like link-local and ULA
• IPv6 Subnetting Guide - Master subnet allocation and learn how to divide your address space efficiently

Frequently Asked Questions#