IPv6

In simple terms

IPv4 addresses are 32-bit numbers (192.168.1.1) — enough for about 4 billion addresses. The internet ran out of them. IPv6 expands to 128 bits (2001:db8::1), giving enough addresses that every grain of sand on Earth could have billions. Beyond sheer space, IPv6 eliminates NAT, simplifies the packet header, and bakes in features that IPv4 retrofitted awkwardly — like stateless address configuration and mandatory IPsec support.

The Visual Map

flowchart TB
  A["2001:0db8:85a3:0042 : 8a2e:0370:7334:0001"] --- B["first 64 bits: network<br/>(global routing prefix + subnet)"]
  A --- C["last 64 bits: interface ID<br/>(SLAAC: self-generated)"]
  subgraph types["address types by prefix"]
    T1["2000::/3 — global unicast (public internet)"]
    T2["fe80::/10 — link-local (auto, this link only)"]
    T3["fc00::/7 — unique local (private, like 192.168.x)"]
    T4["ff00::/8 — multicast (replaces broadcast)"]
    T5["::1 — loopback"]
  end

More detail

An IPv6 address is 128 bits written as eight groups of four hex digits separated by colons: 2001:0db8:85a3:0000:0000:8a2e:0370:7334. Leading zeros in a group can be omitted, and one contiguous run of all-zero groups can be replaced with ::, so the above becomes 2001:db8:85a3::8a2e:370:7334.

Address types:

• Global unicast (2000::/3) — routable on the public internet, the equivalent of public IPv4.
• Link-local (fe80::/10) — auto-assigned on every interface, usable only on the local link; replaces IPv4 APIPA.
• Loopback (::1) — equivalent of 127.0.0.1.
• Multicast (ff00::/8) — one-to-many; replaces broadcast. Router solicitation uses multicast.
• Unique local (fc00::/7) — private addressing, similar to IPv4 RFC 1918 (192.168.x.x).

Key improvements over IPv4:

• No NAT required — every device gets a globally routable address. End-to-end connectivity is restored.
• Stateless Address Auto-Configuration (SLAAC) — a device generates its own address from the network prefix (advertised by the router) and its MAC address or a random value. No DHCP server needed for basic connectivity.
• Simpler header — fixed 40-byte header (vs. variable 20–60 byte IPv4 header). Extension headers replace IPv4 options; routers skip unrecognised extensions.
• Mandatory ICMPv6 — neighbour discovery (replacing ARP), router advertisement, and path MTU discovery are all part of the protocol, not bolted on.
• No fragmentation at routers — only the source can fragment; routers drop oversized packets and send ICMPv6 “Packet Too Big” back. Reduces router work.

Dual-stack deployment: most networks today run both IPv4 and IPv6 simultaneously. Hosts negotiate which to use; DNS returns both A (IPv4) and AAAA (IPv6) records. Browsers use “Happy Eyeballs” — race a connection attempt on each, use whichever connects first.

IPv4 exhaustion is complete — IANA ran out in 2011, regional registries through 2019. NAT patched the shortage at the cost of breaking peer-to-peer connectivity and complicating mobile networks; IPv6 removes all of that, and mobile and IoT deployments are increasingly IPv6-only.

Under the Hood

The notation rules and the network/host split, mechanically:

import ipaddress

a = ipaddress.ip_address("2001:0db8:85a3:0000:0000:8a2e:0370:7334")
print(a.compressed)            # 2001:db8:85a3::8a2e:370:7334  (:: swallows the zeros)
print(a.exploded)              # full form back again
print(f"{int(a):#x}")          # one 128-bit integer underneath

net = ipaddress.ip_network("2001:db8:85a3::/64")
print(a in net)                # True — first 64 bits match
print(f"{net.num_addresses:.3e} addresses in ONE /64 subnet")   # 1.8e19

for addr in ("fe80::1", "::1", "ff02::1", "fc00::42"):
    ip = ipaddress.ip_address(addr)
    print(addr, "->", "link-local" if ip.is_link_local else
          "loopback" if ip.is_loopback else
          "multicast" if ip.is_multicast else
          "unique-local" if ip.is_private else "global")

A single standard /64 subnet holds 4 billion times the entire IPv4 internet — which is why IPv6 design conversations are about prefix delegation, never about conserving addresses.

Engineering Trade-offs

• Abundance vs migration cost. The address space problem is solved permanently, but IPv6 isn’t backwards compatible — every host, router, firewall rule, monitoring pipeline, and hard-coded IPv4 literal needs touching, which is why adoption took decades and dual-stack (running both, doubling operational surface) remains the norm.
• End-to-end reachability vs the NAT comfort blanket. Every device being globally addressable restores peer-to-peer simplicity — and removes the accidental shield NAT provided. IPv6 networks must rely on explicit stateful firewalls; “unreachable by default” is now a choice, not a side effect.
• SLAAC autonomy vs administrative control. Devices configuring themselves with no server is operationally beautiful, but it surrendered the central lease ledger DHCP provided — enterprises wanting per-device tracking re-add DHCPv6 on top.
• Privacy vs stability. Deriving interface IDs from MAC addresses made devices trackable across networks; privacy extensions (random, rotating addresses) fix that but complicate logging, allow-listing, and debugging.

Real-world examples

• Google, Facebook, Cloudflare, and major CDNs are fully dual-stack; their primary address family is IPv6 on mobile networks.
• AWS, Azure, and GCP support IPv6 on all new services; many are dual-stack by default.
• ISPs like T-Mobile US run their mobile network as IPv6-only with IPv4-as-a-service (464XLAT translation).
• IoT devices in smart-home and industrial deployments use IPv6 mesh networks (Thread protocol) where every sensor has a global address.

Common misconceptions

• “IPv6 is not deployed.” As of 2024, IPv6 carries ~45% of Google’s traffic and is the majority of mobile traffic in many markets. Adoption is real and growing.
• “AAAA records are slow.” Browsers use Happy Eyeballs: IPv4 and IPv6 race. If IPv6 is slower, IPv4 wins. The record lookup itself is parallel.

Try it yourself

Check your machine’s IPv6 story — every interface has at least a link-local address, even with no IPv6 internet:

ip -6 addr show | head -10      # fe80:: link-local is always there; global = real IPv6

# requires: network
python3 -c "
import socket
for fam, _, _, _, addr in socket.getaddrinfo('google.com', 443, proto=socket.IPPROTO_TCP):
    print('AAAA' if fam == socket.AF_INET6 else 'A   ', addr[0])
"

If the AAAA lines appear but you can’t reach them, you’re seeing exactly the gap dual-stack and Happy Eyeballs exist to paper over.

Learn next

• IP address — the addressing fundamentals IPv6 extends.
• NAT — the workaround IPv6 was designed to retire.
• DNS — AAAA records, the other half of IPv6 connectivity.
• Router — where router advertisements and prefix delegation happen.