The /64 subnet size is required for SLAAC (Stateless Address Autoconfiguration) — the mechanism that allows IPv6 devices to automatically configure their own IP addresses without a DHCP server. SLAAC works by combining the /64 network prefix with a 64-bit interface identifier derived from the device's MAC address (via EUI-64) or generated randomly (RFC 7217). If the subnet is smaller than /64, there aren't 64 bits for the interface ID and SLAAC cannot function. Since SLAAC is fundamental to IPv6's plug-and-play design, /64 became the mandatory size for all user-facing subnets. Every LAN segment, VLAN, and server network uses exactly /64.

/48, /56, and /64 represent different levels in the IPv6 allocation hierarchy: /48 is a "site" allocation — typically assigned by an ISP to an organisation, providing 65,536 /64 subnets. Large enterprises and data centres receive a /48 (or larger) to address their entire infrastructure. /56 is a "home/residential" allocation — many ISPs assign a /56 to home broadband subscribers, providing 256 /64 subnets — more than enough for home network segmentation (IoT, guest, work, etc.). /64 is a single subnet — every individual network segment (LAN, VLAN, Wi-Fi network) is exactly /64. You never manually divide a /64 into smaller pieces for host addressing.

IPv6 addresses have two simplification rules: (1) Leading zeros in each group can be omitted — 0db8 can be written as db8. (2) One or more consecutive groups of all-zeros can be replaced with :: — but only once per address. To expand: replace :: with the number of 0000 groups needed to make 8 groups total. For 2001:db8::1: count existing groups (2001, db8, 1 = 3 groups), need 8 total, so :: = 5 groups of zeros: 2001:0db8:0000:0000:0000:0000:0000:0001. The /prefix tells you how many of the leftmost bits are the network portion.

No. IPv6 completely eliminates broadcast. Every function that used broadcast in IPv4 is replaced by multicast in IPv6: ARP is replaced by NDP (Neighbor Discovery Protocol) using solicited-node multicast addresses (ff02::1:ff00:0/104), DHCP discovery uses ff02::1:2 (all-DHCP-relay-agents-and-servers), routing protocol hellos use ff02::5 (OSPF all-routers) and ff02::6 (OSPF designated routers), and general "all hosts" is ff02::1. This eliminates broadcast storms and significantly improves performance on large layer-2 segments.

ULA (Unique Local Address, RFC 4193) is the IPv6 equivalent of RFC 1918 private addresses (10.x, 172.16.x, 192.168.x). ULAs start with fc00::/7, with fd00::/8 being the commonly used range. They are not routable on the public internet. Use ULA when: you want private IPv6 addressing for internal services that should never be internet-accessible, you're on a network that doesn't have a globally assigned IPv6 prefix from an ISP, or you're building lab/test infrastructure. For production internet-accessible services, use globally assigned unicast addresses (2000::/3) from your ISP. ULA fd00:: addresses include a pseudo-random 40-bit global ID to minimise conflicts when merging two ULA networks.

Jio (Reliance Jio, ASN 55836) and Airtel (Bharti Airtel, ASN 9498) both have IPv6 allocations from APNIC. Jio's FTTH (JioFiber) and Jio 5G connections increasingly provide IPv6 by default, typically assigning /56 or /60 prefixes to customer routers via DHCPv6-PD (Prefix Delegation). Jio's mobile 4G/5G network uses IPv6 natively with dual-stack. Airtel Xstream Fiber similarly provides IPv6, often assigning a /56 or /48 via DHCPv6-PD. If your Jio or Airtel connection provides IPv6, your router receives a prefix delegation and automatically assigns /64 subnets to each of your home network interfaces. Check your router's IPv6 settings to see your assigned prefix.

EUI-64 is the method SLAAC uses to generate the 64-bit interface identifier portion of an IPv6 address from a device's 48-bit MAC address. The process: (1) Split the MAC address in the middle and insert ff:fe (e.g., MAC 00:1A:2B:3C:4D:5E becomes 00:1A:2B:ff:fe:3C:4D:5E). (2) Flip the 7th bit (Universal/Local bit) of the first byte. (3) The resulting 64 bits become the interface ID appended to the /64 prefix. For privacy, RFC 7217 and RFC 4941 define "privacy extensions" that use a random 64-bit interface identifier instead of the MAC-derived EUI-64, preventing device tracking. Modern operating systems (Windows, Linux, macOS, Android, iOS) use privacy extensions by default.

DHCPv6-PD (Prefix Delegation, RFC 3633) is the mechanism by which your ISP (Jio, Airtel, etc.) automatically assigns a block of IPv6 addresses to your home router. Instead of just giving your router a single IP address, DHCPv6-PD gives it an entire prefix (e.g., a /56) which the router then subdivides into /64 subnets and assigns to each of its internal interfaces (LAN, Wi-Fi 2.4GHz, Wi-Fi 5GHz, guest network, etc.). The process: (1) Router sends DHCPv6 Solicit with IA_PD option requesting a prefix. (2) ISP DHCPv6 server responds with a /56 (or /60) delegation. (3) Router allocates /64 subnets from this delegation to each interface. (4) Devices on each /64 use SLAAC to self-configure their addresses from the /64 prefix.

Yes — completely free, no signup, no account required. All calculations run entirely in your browser using JavaScript BigInt operations for 128-bit precision — no IPv6 address data is transmitted to any server. The calculator supports all IPv6 address formats (full, compressed with ::, mixed IPv4-mapped), all prefix lengths from /0 to /128, and includes a hex nibble visualizer, interactive subnet splitter, preset buttons for common use cases, and copy-per-field functionality.