IPv6 Prefix Delegation (/56 & /60 to /64 VLAN) Calculator
Enter your ISP's delegated IPv6 prefix (such as a `/56` from AT&T/Verizon/Starlink or a `/60` from Comcast Xfinity) to generate clean `/64` SLAAC subnets for your VLANs.
Enter your IPv6 prefix base and delegated size, then click Generate /64 VLAN Subnets.
Why Every IPv6 VLAN Must Always Be Exactly a /64
Engineers accustomed to IPv4 often try to carve a single `/64` into smaller `/68` or `/72` subnets for their Guest and IoT VLANs. In IPv6, breaking a `/64` breaks SLAAC (Stateless Address Autoconfiguration)—meaning Android phones, Google Cast devices, and many IoT sensors will completely fail to obtain an IPv6 address. Instead, request a `/60` (16 subnets) or `/56` (256 subnets) from your ISP via DHCPv6-PD Prefix Hinting so every internal VLAN receives its own standard `/64`.
US ISP IPv6 Prefix Delegation (DHCPv6-PD) Reference Table
| US Internet Provider | Default Prefix Hint | Max Delegated Size | Total /64 VLANs | Router Configuration Note |
|---|---|---|---|---|
| Comcast Xfinity | /64 | /60 | 16 VLANs | Enable 'Send IPv6 Prefix Hint = 60' in router WAN IPv6 settings |
| AT&T Fiber (BGW320) | /64 per request | /60 (via 8x /64 PD) | Up to 8 VLANs | Enable IPv6 Prefix Delegation on BGW320 Home Network > IPv6 tab |
| Verizon Fios | /56 | /56 | 256 VLANs | Native Dual-Stack DHCPv6-PD (`::/56`) works directly from ONT |
| Starlink | /56 | /56 | 256 VLANs | Provides globally routable `/56` IPv6 even though IPv4 uses CGNAT |
| Charter Spectrum | /64 | /56 (Region dependent) | 1 to 256 VLANs | Request `/56` or `/60` prefix hint; falls back to `/64` if unsupported |