Documentation

The Client Subnet EDNS option (RFC 7871 section 6) — three fields: a source prefix length, a scope prefix length, and a (possibly truncated) IP address whose 16-bit FAMILY field is implicit in the constructor's IP value (1 for IPv4, 2 for IPv6).

           +0 (MSB)                            +1 (LSB)
 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 |                          OPTION-CODE                          |
 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 |                         OPTION-LENGTH                         |
 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 |                            FAMILY                             |
 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 |     SOURCE PREFIX-LENGTH      |     SCOPE PREFIX-LENGTH       |
 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 |                           ADDRESS...                          /
 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

The address is masked and truncated to the source prefix length on encode and zero-padded on decode. A FAMILY value other than 1 or 2 fails the decoder (a future revision may decode such values as an opaque option instead).