IPv4 vs IPv6: addresses, dual stack and compatibility

IPv4 and IPv6 are two versions of Internet Protocol that both operate on today’s Internet. IPv4 addresses are 32 bits long; IPv6 addresses are 128 bits long. The larger IPv6 address space does not automatically make a connection faster or safer. Performance and exposure depend on the route, network and configuration.

Address formats and network prefixes

IPv4 is normally written as four decimal numbers, such as 192.0.2.10. IPv6 uses colon-separated hexadecimal groups; a run of zero groups may be abbreviated once with ::, for example 2001:db8::10. Both sample ranges are reserved for documentation, not live public servers. A slash gives the network prefix length: 192.0.2.0/24 and 2001:db8::/32 refer to blocks rather than an individual host. You can explore IPv4 subnet calculations with the 2ip IP calculator.

Why a device shows several addresses

A computer can have a private IPv4 address on its local network, a publicly observed IPv4 address through NAT, an IPv6 link-local address and one or more global IPv6 addresses. With dual stack, an application chooses an available address family. A DNS A record names an IPv4 destination; AAAA names an IPv6 one. A website can publish both, but an AAAA record alone does not prove the server, route and firewall allow IPv6 connections. The IPv6 specification describes the packet format and Hop Limit.

NAT and application compatibility

Because IPv4 space is limited, providers commonly use NAT, and sometimes carrier-grade NAT, so devices share public addresses. IPv6 has far more addresses, allowing devices to hold global addresses without this form of mandatory address sharing. That does not remove the need for firewall rules. An IPv4-only application will not suddenly understand IPv6 because of a DNS change. Communication across address families requires appropriate translation mechanisms or support for both versions at the endpoints.

Troubleshooting one failing version

If a website fails to load, query its A and AAAA records and compare access over IPv4 and IPv6. When only one version works, inspect its address, route, DNS results and firewall rules on both the router and server. Different performance may reflect a different path or network interconnection, not the number of address bits. The 2ip IP information tool reports the address seen for that particular request; another application or request may use another address. Record the time and address family before reporting a fault.