It tells you how many of the 32 bits in your IPv4 addresses are fixed. The rest belongs to your space.

In binary, a /8 netmask would look like this: 11111111.00000000.00000000.00000000 and a /16 would look like this: 11111111.11111111.00000000.00000000

So 3.0.0.0/8 means only the first eight bits are fixed. Which gives Amazon 2^24 IP-Addresses to use (32-8 bit). That is huge because it's 1/256 of all available IPv4 addresses. It is small because it's only 16777216 addresses.

They are subnetwork masks! It's a formal way of expressing in which range you will find the addresses of a given network (for example: 3.0.0.0/8 is like saying "3.x.x.x addresses"; or, for common home networks you will find something like 192.168.1.0/24, which means "192.168.1.x"). I'm not an expert on the subject, but this site seems to have a complete explanation: http://www.steves-internet-guide.com/subnetting-subnet-masks...

I believe it means that this block is 1/2^n of the total address space. Basing this off people discussing how this /8 is made up of two /9s. And there seems to be 256 of the GE sized blocks in the map someone posted above: https://www.caida.org/research/id-consumption/whois-map/imag...

It's called CIDR notation. https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

Go to http://cidr.xyz Try out different combinations