IPv6 Prefix Delegation Calculator

Calculate IPv6 prefix delegation splits. See how a /48 or /56 divides into /64 subnets for your network segments.

Delegated Prefix

Split Into

Figure 1 — IPv6 Prefix Delegation Calculator

What Is IPv6 Prefix Delegation?

IPv6 Prefix Delegation (PD) is the mechanism by which an ISP assigns a block of IPv6 addresses to your router via DHCPv6. Your router then subdivides this block into /64 subnets for each network segment. This is how IPv6 DHCP works at the ISP level — instead of assigning a single IP, your ISP delegates an entire prefix.

Most ISPs assign /48 for business customers and /56 for residential. With a /48, you get 65,536 /64 subnets — more than any home or small business will ever need. Use our IPv6 Subnet Calculator for detailed subnet calculations.

Typical Delegation Hierarchy

Level	Prefix	Assigned By	/64 Subnets
RIR → ISP	/32	IANA/RIR	4.3 billion
ISP → Business	/48	ISP	65,536
ISP → Residential	/56	ISP	256
Router → Subnet	/64	Your router	1

Pro Tip: If your ISP only assigns a /64, you cannot subnet further (SLAAC requires /64). Contact your ISP and request a /56 or /48 delegation. Many ISPs provide larger prefixes on request. Without prefix delegation, you're limited to a single IPv6 subnet. Check your current IPv6 at What Is My IP.

Configuring Prefix Delegation

OpenWrt

# /etc/config/network
config interface 'wan6'
    option proto 'dhcpv6'
    option reqprefix 'auto'

config interface 'lan'
    option proto 'static'
    option ip6assign '64'

Cisco IOS

interface GigabitEthernet0/0
 ipv6 address autoconfig
 ipv6 dhcp client pd ISP-PREFIX
!
interface GigabitEthernet0/1
 ipv6 address ISP-PREFIX ::1/64

Note: DHCPv6 Prefix Delegation is different from SLAAC. SLAAC auto-configures addresses on a single subnet. PD delegates entire prefixes from ISP to router. Your router uses PD to get its prefix, then your devices use SLAAC within each /64 subnet. Configure your router at 192.168.1.1 to request PD from your ISP.

Planning Your Subnet Layout

With a /48 or /56, plan your subnets logically by function. Use our Smart Home Subnet Planner for VLAN-based segmentation and the Network Diagram Generator to visualize your layout.

Subnet ID	Prefix	Purpose
::1::/64	Main LAN	PCs, phones, tablets
::2::/64	IoT	Smart home devices
::3::/64	Guest	Visitor access
::10::/64	Servers	NAS, home lab

Key Takeaways

Prefix delegation assigns entire IPv6 address blocks from ISP to your router via DHCPv6-PD.
A /48 gives 65,536 /64 subnets; a /56 gives 256 — both are more than enough for any home network.
If your ISP only assigns /64, request a larger delegation — you need at least /56 for multiple subnets.
Plan subnets by function: separate LAN, IoT, guest, and server traffic into different /64s.
Use IPv6 Subnet Calculator for detailed calculations and What Is My IP to verify IPv6.

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Frequently Asked Questions

What is IPv6 prefix delegation?

Prefix delegation is how ISPs assign IPv6 address blocks to customer routers via DHCPv6. The router subdivides the block into /64 subnets for each network segment.

How many /64 subnets from a /48?

A /48 provides 65,536 /64 subnets. A /56 provides 256 subnets.

What if my ISP only gives me a /64?

You cannot subnet further. Contact your ISP to request a /56 or /48 delegation. Many ISPs provide larger prefixes on request.

Do I need prefix delegation for a home network?

If you want multiple IPv6 subnets (for VLANs, IoT isolation, guest network), yes. A single /64 limits you to one subnet.

How do I check my delegated prefix?

Log into your router and check the WAN IPv6 status. It should show the delegated prefix and length. Or check What Is My IP for your IPv6 address.

About Tommy N.

Tommy is the founder of RouterHax and a network engineer with 10+ years of experience in home and enterprise networking. He specializes in router configuration, WiFi optimization, and network security. When not writing guides, he's testing the latest mesh WiFi systems and helping readers troubleshoot their home networks.