In a TCP stack sequence and acknowledgement, fields are 32 bit [4 Byte]. There are 2^32= 4,294,967,296 possibilities of generating a random number:
Between a client and a server, when a client initiates a connection it generates an initial sequence number (ISN). Every OS uses its own algorithm to generate an ISN, and this is pretty much predictable.
Random ISNs are good because they make it far harder to predict sequence numbers. In Windows 2003 onward, the ISN algorithm has been modified so that ISNs ...