networking - Can you grab any IP address on the Internet?

On a personal network (LAN) one can simply grab an IP address. If you choose the same IP address as an existing client, you get problems. There are companies like IANA and ICANN that are in charge of IP address bulks and sells them. But what stops you from just grabbing a random IP address? Is this built on trust? What if somebody were to grab an IP address and conflicting would occur. Is there any way of tracing that IP address to the location of the server using it?

Are companies that actually maintain the physical internet cables checking whether or not the clients connecting are using bought IP address blocks?

Answer

There is nothing preventing you from attaching a box configured with someone else's IP address to the internet. However, this won't necessarily cause any issues for anyone else but yourself.

If you steal someone else's IP address outside of the subnet that you are physically connected to, the only thing you will accomplish is not being able to receive any traffic as any router, behaving properly, is going to route traffic to the real owner of this IP address. You might be able to advertise false routes to whatever edge router is upstream from you in the hopes that they would get propagated further in the hopes of getting traffic routed to you based on your stolen IP address, but any marginally competent ISP/upstream provider would never accept routes from its non-enterprise consumers. As far as enterprise customers/other ISPs go, they are bound by specific rules about what routes they can advertise and use with their transit provider or peer, which are monitored 24/7 by Network Operations and Control Teams. Most also have rules about what routes they will accept as valid depending on who advertised them. In short, stealing someone's IP address outside the subnet you are connected to does nothing unless you can also manipulate the upstream routing tables.

That aside if you were to steal an IP address of someone in your same subnet, you would disrupt traffic of both the person who owns it and yourself. With any managed switch or router this will raise alarms as there is a duplicate address on the network and will likely lead to your connection getting blocked in some way.