I found this that may be helpful port forwarding for your router
on your question
base host port is the port you are opening on your router which means that whatever communication stablished with an outside-your-network device on x-y port range will come through into or out of your router through that base host port.
for example:
so if you have an application that works with ports abc to xyz, any communication that comes out of your router that is between that range will go out of the base host port abc (the lower number in the range), in the same way if there's info going out through and out of your router to any port between abc and xyz range it will go through port abc, your host base port.
Answers & Comments
You are probably doing some port forwarding..
I found this that may be helpful port forwarding for your router
on your question
base host port is the port you are opening on your router which means that whatever communication stablished with an outside-your-network device on x-y port range will come through into or out of your router through that base host port.
for example:
so if you have an application that works with ports abc to xyz, any communication that comes out of your router that is between that range will go out of the base host port abc (the lower number in the range), in the same way if there's info going out through and out of your router to any port between abc and xyz range it will go through port abc, your host base port.
hope that helps.
I hope that makes sense