Load balancing two connections - which IP address?

I'm considering a load balancing router to pair an ADSL2 line with a 4G connection (or possibly 5G - according to the coverage maps we're just on the boundary of 5G)

The ADSL line is only 12/1mb but has a static IP. My question is, which IP address does the outside world see? The static one or the 4G IP?

I'm new to load balancing so any pointers would be helpful!

TIA

In reply to a post by simonh__:

… which IP address does the outside world see? The static one or the 4G IP?

If you visit a site which reports your public IP, it sees whichever connection the load balancing router used for that traffic on the way out.

The algorithm will usually have options for varying degrees of stickyness. For example traditionally when establishing a TLS connection (e.g. https) it would send subsequent related traffic out through the same link until the client closed the connection. The way websites and online services use CDNs these day, it probably isn't that clear cut.

If you are thinking about port-forwarding, while you can add the same rules to multiple connections, it will likely only make sense to come in on the static IP connection. Even with dynamic DNS it is not guaranteed that the the 4G/5G connection will always have a routable public IP for inbound connections as compared with an IP shared amongst other subscribers.

If either ISP supports dual-stack w/ IPv6 your options change again.

Edited by prlzx (Sun 31-Jan-21 12:53:53)

Sorry I've just seen this, thanks for the reply