There is one option under General Settings: Enable Consistent NAT.
Consistent NAT enhances standard NAT policy to provide greater compatibility with peer-to-peer applications that require a consistent IP address to connect to, such as VoIP. Consistent NAT uses an MD5 hashing method to consistently assign the same mapped public IP address and UDP Port pair to each internal private IP address and port pair.
For example, NAT could translate the private (LAN) IP address and port pairs, 22.214.171.124/50650 and 126.96.36.199/50655 into public (WAN) IP/port pairs, as shown in IP address and port pairs.
IP Address and Port Pairs
Translated Public IP/Port
With Consistent NAT enabled, all subsequent requests from either host 188.8.131.52 or 184.108.40.206 using the same ports illustrated in IP address and port pairs result in using the same translated address and port pairs. Without Consistent NAT, the port and possibly the IP address change with every request.
Enabling Consistent NAT causes a slight decrease in overall security, because of the increased predictability of the address and port pairs. Most UDP-based applications are compatible with traditional NAT. Therefore, do not enable Consistent NAT unless your network uses applications that require it.
For Consistent NAT to work properly, the minimum time interval between calls must be at least 200 msec.
Enabling Consistent NAT
To enable consistent NAT
Select the Enable Consistent NAT option. This option is not selected by default.