what should I input in the ip field to give access to 123 port in the router

Hi, There is only one port 113 is shown as accessible port in the router settings. For time syncing purpose, I had to input some ip value to the blank place.
I am using Dlink 2750u router, and getting access to the pc over wifi. To manage router settings , I use 192.168.1.1 as router ip. When I filled up this 123 as customized port, and input this ip, I get a error message, that this is the ip of the modem. Please input valid input for Lan.
I do not know which ip has to be given here to give access to the ntp server port udp 123.
Would you please advise me.

There is no reason to port forward ntp.

Ok. I enabled accesss to port 123 in my mcafee firewall settings. But this port is not shown as open ports . I think, that this port is to be in open port, if ntp services are to be allowed.