Draytek Vigor2860 Series User Manual page 408

5. Open NAT >> Address Mapping. Pay special attention to the Mask setup.
6. Click Index number 1 and 2 to configure the details.
Here the Private IP can be any IP address within the range of 192.168.1.16 through
192.168.1.31. The Subnet Mask defines the size of the IP range, and the Private IP is an
indicator of the IP range. Therefore, the combination of Private IP and Subnet Mask
determine the IP range.
7. Upon completing the above configuration, you have specified the outgoing IP address(es)
for some specific computers. But you still have to specify the outgoing interface for them.
Otherwise, the traffic may be routed out with IP address 172.17.1.55 or 172.17.1.57
through WAN2 port. The load-balance policies are below.
Vigor2860 Series User's Guide 398