Why would these spam marketing companies want to buy IPv4 addresses? Would IPv6 not work? Or were they trying to assume the identity and reputation of the companies that owned the IPv4 addresses previously?
Everybody on the internet has access to IPv4, the same is not true of IPv6 as of the end of 2019. IPv6 addresses are also pretty easy to get without doing shady deals.
I'm not sure how tolerated this is on HN, but I'm lazy to research background on this. Could you please explain what a /64 is and why it is significant in this regard?
A netblock. A "/n" means "a block of IP addresses where the first n bits are the same".
This comes from the CIDR notation 192.168.1.0/24 indicating the netblock from 192.168.1.0 - 192.168.1.255.
So the number after the slash corresponds to a network size. The smaller the number, the more bits are variable, i.e. the bigger the netblock.
An IPv4 /24 has 32 (address length) - 24 = 8 bits that are variable, i.e. 2^8 addresses. A /18 would have 32-18 = 14 variable bits, i.e. 2^14 addresses.
In IPv6, the address length is 128. A /64 (which happens to be the smallest routable network size in IPv6) is 2^64 addresses - but because it's easy to get large IPv6 netblocks (typical ISP allocations are /64, /56, and you can often get a /48 with relative ease), for abuse blocking purposes, you generally treat the entire /64 or even /56 as one entity (i.e. you ignore the rightmost 64 or 72 bits).
A /56 is 2^8 separate /64s. A /48 is 2^16 separate /64s.
Since none of us know exactly how much you already know about IP routing, we’ll likely either waste time providing an answer that assumes knowledge you don’t yet have, or waste time providing extraneous detail you don’t need.
I suggest googling something along the lines of “ipv6 allocation 56 64“ sans quotes.
[Edit: never mind; the HN community seems quite generous with their time today.]
The problem is that thinking for someone else doesn't "teach them to fish (learn independently)" and "be the change you seek," rather it enables co-dependency on others.
This is smart, would love if it got smart enough to analyze .dockerignore files, and (if you're analyzing our code with the rest of your tool anyway) you could find large files/directories that we COPY/ADD and don't need