anchor.diff:            (let mask =
                   if i.prefixLength != null then toString i.prefixLength else
                   if i.subnetMask != "" then i.subnetMask else "32";
  +               staticIPv6 = cfg.enableIPv6 && i.ipv6Address != null;
             in
             { description = "Configuration of ${i.name}";
               wantedBy = [ "network-interfaces.target" ];
  
positive.code:                     echo "configuring interface..."
                      ip -4 addr flush dev "${i.name}"
                      ip -4 addr add "${i.ipAddress}/${mask}" dev "${i.name}"
                      # Ensure that the default gateway remains set.
                      # (Flushing this interface may have removed it.)
                      ${config.systemd.package}/bin/systemctl try-restart --no-block network-setup.service
                    else
                      echo "skipping configuring interface"
                    fi
                    ${config.systemd.package}/bin/systemctl start ip-up.target
                  ''
  
negative.code:           Subnet mask of the interface, specified as a bitmask.
            This is deprecated; use <option>prefixLength</option>
            instead.
          '';
        };
  
        ipv6Address = mkOption {
          default = null;
          example = "2001:1470:fffd:2098::e006";
          type = types.nullOr types.string;
          description = ''
            IPv6 address of the interface.  Leave empty to configure the
            interface using NDP.
          '';
        };
  
        ipv6prefixLength = mkOption {
          default = 64;
          example = 64;
          type = types.int;
          description = ''
            Subnet mask of the interface, specified as the number of
            bits in the prefix (<literal>64</literal>).
          '';
        };
  
  