You used to be able to buy blocks of public addresses from ARIN (and I guess you still can[0] but only under special circumstances[1]) but the IPv4 public assignment table is essentially full.  You could buy blocks in class A (i.e you would own X.&lt;all&gt;.&lt;all&gt;.&lt;all&gt;), class B (X.Y.&lt;all&gt;.&lt;all&gt;), or class C (X.Y.Z.&lt;all&gt;).<p>The class assignments themselves are now either a result of an RFP (for special class IP blocks like multicast) or historical significance (basically, who grabbed it when it was available).  XKCD had a comic in 2006 mapping the IPv4 assignment table[2].<p>I don&#x27;t know much about IPv6 other than that its address space is so large that you essentially don&#x27;t need to use NAT anymore, so I doubt we&#x27;ll ever deplete it.<p>0. <a href="https:&#x2F;&#x2F;www.arin.net&#x2F;fees&#x2F;fee_schedule.html" rel="nofollow">https:&#x2F;&#x2F;www.arin.net&#x2F;fees&#x2F;fee_schedule.html</a><p>1. <a href="https:&#x2F;&#x2F;www.arin.net&#x2F;resources&#x2F;request&#x2F;ipv4_countdown.html" rel="nofollow">https:&#x2F;&#x2F;www.arin.net&#x2F;resources&#x2F;request&#x2F;ipv4_countdown.html</a><p>2. <a href="https:&#x2F;&#x2F;xkcd.com&#x2F;195&#x2F;" rel="nofollow">https:&#x2F;&#x2F;xkcd.com&#x2F;195&#x2F;</a>