Companies can rent address spaces, and use these addresses dynamically or statically to assign it to their network participants.
Some areas can not be rented, and are called private addresses.
These are the address ranges:
> 10.0.0.0 - 10.255.255.255 -- 16777216 addresses > 172.16.0.0 - 172.31.255.255 -- 1048576 addresses > 192.168.0.0 - 192.168.255.255 -- 65536 addresses
These ranges give everyone the opportunity to work with them and establish a local network. As packets with private addresses are not routed to the public net, there is a natural shielding and a local network can not collide with another.
It does not matter whether someone somewhere else uses the same addresses in its local network, because the local networks can not be directly superimposed.
Shall packages be transferred from a local network to another, they must pass through the Internet. You’ll need an external IP address for this. Your external IP address is the one you see on this site.
This address has nothing to do with your internal IP address, that you have assigned to your computer manually of via DHCP.
The transition local/public and thus from internal to external IP address is done by the router, that does a NAT (address translation, network address translation).
If you want to take a number of networks together, it is possible with VPN (virtual private network).