IPv4 vs IPv6
The differences between the two IP versions that actually matter in day-to-day network engineering — addressing, configuration, NAT, headers and more, without vendor marketing.
Side-by-side comparison
| Attribute | IPv4 | IPv6 |
|---|---|---|
| Address size | 32 bits | 128 bits |
| Notation | 192.168.1.1 (four dotted-decimal octets) | 2001:db8::1 (eight hex groups, compressible) |
| Total addresses | 4,294,967,296 | ≈3.4 × 1038 |
| Configuration | Manual/static or DHCP | SLAAC (stateless autoconfiguration) or DHCPv6 |
| NAT | Ubiquitous, largely because public addresses are scarce | Not required by design; end-to-end addressing is the norm |
| Header | Variable length (20–60 bytes), includes a checksum | Fixed 40-byte header, no IP-layer checksum |
| Broadcast | Supported | No broadcast; multicast instead, including efficient solicited-node multicast for neighbor discovery |
| Fragmentation | By the sending host and intermediate routers | By the sending host only; routers rely on Path MTU Discovery |
In raw capacity terms, IPv6 offers about 7.9 × 1028 times as many addresses as IPv4. The two notations are not interchangeable: an IPv4 address never contains letters or colons, and IPv6 uses prefix lengths (/64) rather than dotted masks.
Why IPv6 exists at all
IPv4's 4,294,967,296 addresses once seemed inexhaustible. They were not: IANA distributed its last central blocks in 2011, and every regional registry has since exhausted its own free pool. The short-term patches — NAT, carrier-grade NAT, address leasing markets — work, but they add complexity and break the original end-to-end model of the internet. IPv6's 128-bit space removes the scarcity that made those workarounds necessary.
Where you still need IPv4 today
- Legacy systems. Old appliances, industrial controllers, printers and embedded devices often have no IPv6 stack at all.
- Enterprise software assumptions. Firewall rules, ACLs, monitoring dashboards and licensing schemes are frequently written against IPv4 syntax first — if at all.
- Carrier environments. Mobile and consumer ISPs commonly run IPv4 over carrier-grade NAT (the 100.64.0.0/10 range), so customer-facing troubleshooting still starts with v4.
- Interoperability. Until every peer, resolver and path speaks v6, dual-stack networks must keep full IPv4 routing, DNS and security tooling alive.
The practical takeaway: learning IPv4 subnetting is not optional, even on networks with healthy IPv6 deployment. Our IPv4 subnet calculator and VLSM calculator cover that side.
The dual-stack reality
Most transitioned networks run both protocols side by side: every interface gets an IPv4 address and one or more IPv6 addresses, and applications use whichever the DNS answer suggests (A records for v4, AAAA records for v6). That means double the addressing plan, double the firewall policy surface and double the troubleshooting matrix — but it is how real networks operate during the long coexistence period.
When you do get a native IPv6 allocation, the planning math changes shape: you typically receive a /48 or /56 and slice it into /64 LANs. Work out the details with the IPv6 subnet calculator, or read the conventions behind the most common prefix on the /64 reference page.
Related tools and references
- IPv6 subnet calculator — expand, compress and analyze any IPv6 prefix.
- IPv6 /64 reference — why the standard LAN subnet holds 264 addresses.
- Subnet cheat sheet — all IPv4 prefixes from /0 to /32, printable.
- CIDR notation explained — the slash-prefix syntax both protocol versions share.