Public Address Range Question

Hello all,

One of our networking guys is out this week and I was trying to take a look at one of the tickets we would usually send his way.Basically we have a customer that owns an address range on a /29 subnet and we need to make a port forwarding change.

So for example sake, I'll use 200.1.1.0/29 as the address range.

Currently 200.1.1.1 would be the gateway/ISP device,200.1.1.2 would be their existing router/firewall.

They own 200.1.1.3 through 200.1.1.6 but they are not in use currently.We need to move a port forwarding rule setup on 200.1.1.2 to another usable IP address.

My question is, lets say if we use 200.1.1.3 for this, does that IP not need to be applied on an interface on their router? 200.1.1.2 is configured on an interface. But if I move the forwarding to 200.1.1.3, wouldn't that still need to be applied on an interface on the router so that it knows how to get there?

I am not a networking guy so don't bust me too hard on this haha. Also I'm sorry if this breaks any of the channel rules, I don't think it does but just in case I missed one.

One of my coworkers said I shouldn't have to because the address range is configured from the ISP and it should still route to that device but I am a little confused on how. If someone could break this down I'd greatly appreciate it.

Thanks!