Networking · Reference
IPv4 vs IPv6: Complete Comparison
IPv4 has been the internet's addressing protocol since 1983. IPv6 was standardised in 1998 and is now used by roughly half of all internet traffic. This guide is a side-by-side reference of every meaningful difference between the two — written for engineers who need a single authoritative comparison they can cite.
The 10-second summary
IPv4 uses 32-bit addresses (4.3 billion total, mostly exhausted) and relies on NAT to stretch the address space. IPv6 uses 128-bit addresses (3.4 × 10³⁸ total) and gives every device a globally routable address, eliminating the need for NAT in most cases. The two protocols are not compatible at the wire level, so hosts run them in parallel ("dual stack") during the long transition period.
Comparison table
| Feature | IPv4 | IPv6 |
|---|---|---|
| Address size | 32 bits | 128 bits |
| Total addresses | ~4.3 × 10⁹ | ~3.4 × 10³⁸ |
| Notation | Dotted decimal (192.0.2.1) | Hex, colon-separated (2001:db8::1) |
| Header size | 20–60 bytes (variable) | 40 bytes (fixed) |
| Header checksum | Yes | No (offloaded to L2/L4) |
| Address configuration | Manual or DHCP | SLAAC, DHCPv6, or manual |
| NAT required | Almost always (CGNAT, home NAT) | Not normally needed |
| Broadcast | Supported | Removed — replaced by multicast |
| Multicast | Optional | Built-in, mandatory |
| Fragmentation | Routers or sender | Sender only (PMTUD) |
| IPsec | Optional | Implementation mandatory (use optional) |
| Address scopes | Private / public | Link-local, unique local, global |
| DNS record | A | AAAA |
| Loopback | 127.0.0.1 | ::1 |
| Adoption (2026) | 100% of legacy infra | ~50% of Google traffic |
Address space, in human terms
IPv4's 4.3 billion addresses sounded infinite in 1983, when the global population was 4.7 billion and there were fewer than a million computers online. By 2011, IANA had allocated the last unallocated /8 block to the regional registries; today, IPv4 addresses are bought and sold on a secondary market for roughly $30–$50 each.
IPv6 gives each /64 subnet 18 quintillion addresses (more than the entire IPv4 internet, squared). The standard recommendation is to give every customer site a /48 — that's 65 536 subnets per home, far more than any household will use, but the wastefulness is deliberate: routing aggregation matters more than density.
Header format and why it matters
IPv6's header is a fixed 40 bytes — twice the typical IPv4 size, but with no options field, no checksum, and no fragmentation fields. Routers can forward IPv6 packets without rewriting the header, which is one reason carrier hardware can route IPv6 at line rate with simpler ASICs. Anything optional (fragmentation, source routing, IPsec) moves into "extension headers" that only endpoints need to parse.
NAT, port forwarding, and end-to-end connectivity
Under IPv4, your home router shares one public address among every device using NAT. That works for outbound browsing but breaks anything inbound: incoming SIP calls, game lobbies, peer-to-peer apps, self-hosted servers. Carrier-grade NAT (CGNAT), used by most mobile networks, breaks even some outbound protocols.
IPv6 restores the original end-to-end model: every device has a public address, and the firewall — not the translator — decides what's allowed in. If you run anything inbound (a self-hosted Plex, Home Assistant, game server, or Matrix homeserver) IPv6 makes life dramatically easier.
Security: not better, not worse, just different
IPv6 doesn't change the threat model. The address space is too large to scan exhaustively (scanning a single /64 at one billion probes per second would take 585 years), so blind reconnaissance is impractical — but attackers don't blind-scan in 2026 anyway, they use leaked DNS data, certificate transparency logs, and BGP information. Every IPv6 host still needs a host firewall and proper patching.
When to use which
- Dual stack everywhere it's practical. Run both in parallel; the operating system picks IPv6 when available and falls back to IPv4 automatically (Happy Eyeballs, RFC 8305).
- IPv6-only inside the data center. Major cloud providers (AWS, Google, Hetzner) now charge extra for IPv4. IPv6-only Kubernetes clusters with NAT64 for outbound IPv4 are now common.
- IPv4-only is fine for legacy LAN devices. A printer from 2014 doesn't need IPv6.
Try it on Toolzer
- What Is My IP shows both your IPv4 and IPv6 address, side by side.
- IP Lookup resolves geolocation, ASN, and ISP for any v4 or v6 address.
- Subnet Calculator handles both IPv4 CIDR and IPv6 prefix math.
- DNS Lookup returns A and AAAA records together.
Frequently asked questions
Is IPv6 faster than IPv4?+
Not inherently. IPv6 removes some per-packet overhead (no header checksum, simpler header) and avoids NAT traversal, which can reduce latency on connections that would otherwise be NAT'd. In practice, the speed difference comes from the path your traffic takes, not the protocol itself. Major CDNs serve IPv6 and IPv4 at the same speed.
Do I need IPv6 if my ISP gives me IPv4?+
For browsing, no — almost every public site is reachable over IPv4 thanks to carrier-grade NAT. But if you run a server, host game lobbies, or use peer-to-peer apps, IPv6 gives you a real public address without NAT, which avoids port-forwarding and double-NAT headaches.
Why are IPv6 addresses so long?+
IPv6 uses 128 bits per address (versus 32 in IPv4), giving 3.4 × 10³⁸ addresses — enough to assign trillions of addresses to every device on Earth. The length is the entire point: it permanently solves the address-exhaustion problem that forced IPv4 into NAT.
Can IPv4 and IPv6 talk to each other directly?+
No. The two protocols are incompatible at the wire level. Dual-stack hosts run both at once, and translation technologies (NAT64/DNS64, 464XLAT) let IPv6-only clients reach IPv4-only servers via a translator.
What percentage of the internet uses IPv6 today?+
As of 2026, roughly 45–50% of users reach Google over IPv6 (per Google's public statistics), with adoption above 70% in India, the United States, Germany, and France. Server-side adoption is lower — many smaller sites are still IPv4-only.
Is IPv6 more secure than IPv4?+
The protocol itself doesn't change the security model. IPv6 makes IPsec mandatory to implement (but not to use), and the huge address space makes random scanning impractical. On the other hand, every IPv6 host typically has a globally routable address, so firewalls and host hardening matter more than under NAT'd IPv4.
Sources: RFC 791 (IPv4), RFC 8200 (IPv6), Google IPv6 Adoption Statistics (2026), IANA IPv4 Address Space Registry.