6rd Address Tool¶

Compute the customer-delegated IPv6 prefix produced by a 6rd service-provider domain, defined by RFC 5969. The tool also runs the reverse: given a 6rd IPv6 address and the SP parameters, it extracts the customer's IPv4.

Service-provider configuration required

Unlike 6to4 (which uses the well-known 2002::/16 prefix and embeds the full IPv4), 6rd uses the SP's own globally-routed IPv6 prefix. The SP IPv6 prefix and SP IPv4 mask length are out-of-band configuration — typically delivered to CPE via DHCPv6 OPTION_6RD. The tool cannot guess them from the address alone, so this tool does not auto-detect 6rd from the embedded-v4 detector.

How a 6rd delegation is constructed¶

The customer-delegated IPv6 prefix is the concatenation of:

The SP IPv6 prefix (sp_ipv6_prefix, e.g. 2001:db8::/32).
All but the first sp_ipv4_mask_len bits of the customer IPv4 address. The masked-off bits are constant across the SP's footprint and therefore not embedded.

The new prefix length is sp_prefix_length + (32 - sp_ipv4_mask_len).

Worked examples¶

Full v4 (mask length 0)¶

Field	Value
SP IPv6 prefix	`2001:db8::/32`
SP IPv4 mask len	`0`
Customer IPv4	`192.0.2.1`
Delegated prefix	`2001:db8:c000:201::/64`

The full 32 bits of the customer v4 follow the /32 SP prefix, giving a /64 delegation.

Truncated v4 (mask length 8)¶

Field	Value
SP IPv6 prefix	`2001:db8::/32`
SP IPv4 mask len	`8`
Customer IPv4	`192.0.2.1`
Delegated prefix	`2001:db8:2:100::/56`

The top 8 bits of the v4 (0xC0) are discarded; the remaining 24 bits (0x000201) are appended to the /32 SP prefix, yielding a /56 with the customer-contributed bits packed at IPv6 bits 32..55.

Decoding (6rd address → customer IPv4)¶

The decoder verifies the address starts with the SP prefix bits, then reads the customer-contributed bits from the position immediately following. The masked-off region (top sp_ipv4_mask_len bits of the v4) is treated as zero — that portion is SP-side configuration and cannot be recovered from the address alone.

Shareable URLs¶

The drawer state is fully captured in the URL:

/ipv6/6rd?sixrd_mode=encode&sixrd_sp_prefix=2001:db8::/32&sixrd_mask_len=0&sixrd_ipv4=192.0.2.1
/ipv6/6rd?sixrd_mode=decode&sixrd_sp_prefix=2001:db8::/32&sixrd_mask_len=0&sixrd_ipv6=2001:db8:c000:201::1

REST API¶

POST /api/v1/6rd mirrors the UI. See the REST API page or the OpenAPI spec at /api/openapi.yaml for the full request/response schema.

curl -X POST https://example.com/api/v1/6rd \
  -H 'Authorization: Bearer YOUR_TOKEN' \
  -H 'Content-Type: application/json' \
  -d '{"mode":"encode","sp_ipv6_prefix":"2001:db8::/32","sp_ipv4_mask_len":0,"ipv4":"192.0.2.1"}'