The answer is NAT - Network Address Translation. Your home network probably has IP address beginning with 192.168.1.* or 192.168.0.* This range, and a few others, are officially reserved and not allowed on the greater internet. They are intended to be allowed for use in private networks - like your own home. Your router's job is to receive 1 IP address for your whole home, and allow connections to the internet to be made using this IP address instead. It will save to its memory what connections are in progress, which device in your home is actually using it, and rewrite IP address on the way in and out of your home. You think you have IP address 192.168.1.55 but your public IP address might be 192.0.2.77. The router swaps these back and forth as data passes through it. Thus every home needs only 1 IP address. In some situations, even the ISP does some NAT stuff as well. Your "public" IP address might come from this special reserved pool and your ISP runs NAT, allowing dozens or hundreds of customers to share 1 IP address. This happens all over the place. Chances are your cellular phone on mobile data is getting NAT'd to be one of many thousand devices on a single IP address. A big company office only needs 1 IP address most of the time. Even if a company does buy large blocks of IP addresses, sometimes they don't want every computer accessible and will do partial NAT on some of them, and give others real internet connections. This is why "port forwarding" is a thing for most home users sometimes. If someone from the internet tries to connect to you, your router has no idea which device the incoming connection belongs to. It must be told by configuring it with "port forwarding" rules, telling it how to identify incoming requests and which device they will belong to. Eg: 192.168.1.32 is my web server, for TCP port 80. Well now new port 80 connections will be re-written to 192.168.1.32 on the incoming path. More on reddit.com

A good question! To understand how it is possible, we must consider two things: Private addresses. These are addresses / networks which the IETF (Internet Engineering Task Force) has designated as reserved in the standards (RFC1918 for IPv4). Those addresses are intended to be used in environments where hosts are not supposed to be globally routable (hence the name "private"). As you can imagine, they were first defined to address the address space exhaustion, but they also exist in IPv6. By defining private networks, what one can do is "hide" an entire network like 192.168.0.0/16 (basically, 65536 hosts) behind a single globally routable address - the router, then, must have a technique to determine how to route packets between the "internet" and hosts in the private network. 2. One of those techniques is Network Address Translation (NAT). NAT itself defines many ways in which we can map multiple hosts (on the private network) to one (on the "outside") - a commonly encountered solution is port address translation (roughly speaking, assigning a different port for each host - 40.1.2.100:3643 for host A, 40.1.2.100:3644 for host B, etc. where 40.1.2.100 is the globally routable, external facing IP address). There are many ways to implement NAT and routing traffic through it (NAT traversal). For a good summary, I suggest you read the Wikipedia article. More on reddit.com

From the groups that distribute them they have been out for a while. But the next level down (ISPs and so forth) still have a decent supply and there is a secondary market for IPs people have they are not using. More on reddit.com

The world ran out of new IPv4 allocations years ago. Now IPs are pretty much bought/sold/traded as needed, with many organizations utilizing NAT or IPv6 (like residential ISPs) to alleviate the need for more IPv4 addresses. More on reddit.com