How to access L2TP VPN server behind ASUS n16 router?

Question

VPN

There is a computer on the local network on which L2TP VPN is configured (Mac OS Server). It is connected to an asus n16 router. How can you access it from outside? As I understand it, you need to forward some ports on the router to this computer, but which ones?
1723 tcp/udp, 1701 udp not enough

Answer 1

According to IANA (and I tend to believe them), the protocol needs ports 1701 for the udp and tcp
protocols to work. And ports 1723 are used for something else .

Answer 2

L2TP over IPSec
L2TP traffic – UDP 1701
Internet Key Exchange (IKE) – UDP 500
IPSec Network Address Translation (NAT-T) – UDP 4500
Allows IPsec Encapsulating Security Payload (ESP) traffic to the VPN server - UDP 50
In general, you can read here :
technet.microsoft.com/en-us/library/dd458955(v=ws....
Do not forget that if the server is still behind NAT, then you need to register a key in the registry to connect behind NAT.