Let's say I have a network to design and a portion of it is far away, say in another city. I bought or rent a wired connection. How should be my subnetwork configuration, do I need to use a subnetwork or IP for the wire(!) I used? Also, can someone explain what this long range cable(!) is? Thanks.
Disclaimer: I'm assuming you're acquiring a private a point-to-point link.
If I understand you correctly, you're asking if you need an IP address on each end of the new connection. That depends on whether you are bridging (Layer-2) or routing (Layer-3) this connection.
Typically, a fibre provider will leave a fibre-to-ethernet converter that simply bridges the connection. If you want to keep the new location as an extension of your existing local network, you don't need to do anything (other than perhaps some switch config and merging the two networks into one subnet).
However, if you are routing the two networks (both on different subnets), you'll need an intermediary subnet for both end devices connected to the fibre, typically a /30, like this:
local LAN (10.0.1.0/24)->fibre int(192.168.0.1/30)->NEW_FIBRE->fibre int(192.168.0.2/30)->remote LAN (172.16.1.0/24)
Then on the local side, you'd have a route pointing 172.16.1.0/24 to 192.168.0.2, and on the remote side, 10.0.1.0/24 to 192.168.0.1.
Note that it's often beneficial to have an address on the external side of any connection for troubleshooting purposes.