What is the "24" in 255.255.0.0/24?

Question

As per title, bit also what do different numbers mean?

I don't know what the name is of that number - feel free to improve the title.

Answer 1

The address you give is in what is called "Classless Internet Domain Routing" format, or sometimes "supernetting". Originally, internet addresses were allocated in fixed-size blocks, and the size of the block and the first few bits of the block's address defined the class. In 1993, because of improvements in technology and a perceived need for smaller and more varied block sizes (because IP addresses were being used at a rapid rate), the Class-based allocation (described at https://en.wikipedia.org/wiki/Classful_network) was replaced by the current Classless allocation (described at https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing).

The number before the slash (255.255.0.0 in your example in the title) is an address or a netmask; the number after the slash (/24, in your example) defines the block via the netmask - in other words, your title defines a net block of 256 addresses in the range 255.255.0.0 through 255.255.0.255 - the first 24 bits define the constant part of the block, and the remainder can vary per device.

Both numbers are important; if you had written (for example) 10.4.0.0/16, the block defined would be much larger, varying from 10.4.0.0 to 10.4.255.255, since only 16 bits would be defining the constant part of the block. The two links give a much better explanation than I can.