What Specifically Happens When You Add a Static IP to a Computer in a Subnet?

Question

This question may be too naive to be Google-able, but I haven't found anything.

I'm curious what happens when I go in to a Client device like my personal Laptop and add a Static IP (as opposed to doing so via the Router itself).

How does the Router learn about this new IP? Does something with Routing Tables change that get propagated (not sure if my Laptop would have one of its own, I'd assume not, and just Gateway)?

Thanks!

EDIT:
I also wish to add that I'm considering to create my own DNS Server (for learning purposes), and intend to give it a Static IP (via netplan).

So please also consider Server-specific Static IP cases (which I understand tend to be one of the main use-cases for Static IPs).

Answer 1

How does the Router learn about this new IP?

Only when some packet originating from that IP hits the router, ie. when it tries to get forwarded to another network, or when there are additional functions on the router that are addressed by the packet.

However, if the new IP doesn't belong to the router's subnet (on that interface), the above won't happen.

Does something with Routing Tables change that get propagated?

No. Routing tables don't change by themselves. Entries are either automatic (for subnets directly attached to the router), static (manual addition by an admin), or learned through a routing protocol like OSPF or BGP.

What you use the new IP address for is irrelevant. If it's a private IPv4 address intended to be accessible from the public Internet you'd need destination NAT aka reverse NAT aka port forwarding, of course.

Answer 2

It depends.

If you configure an address in the same subnet as the router interface, AND you configure the gateway address to be the router interface address, everything will work just fine.

If you do anything else, then you will likely fail to communicate with anything.

If your router interface doesn’t have an address, then nothing will work.