Target IP addresses are assigned so that the addresses of all hosts on a given network share a common prefix. The common prefix defines the network portion of the IP address, and the remainder defines the host portion (also referred to as the local portion).
The term network in this context refers to a logical network which might span one or more physical networks. The network portion of an IP address identifies a site and the local portion identifies a single host at that site.
A site using subnet addressing must specify a 32-bit subnet mask for each network. Each bit in the subnet mask is set to 1 if the network treats the corresponding bit in the IP address as part of the network address or 0 if it treats the corresponding bit in the IP address as part of the host ID.
Consider, for example, the subnet mask
11111111 11111111 0000000 0000000
(or in decimal form, 255.255.0.0). This subnet mask specifies that the first two octets identify the network and the last two octets identify the host on that network.
The subnet mask 255.255.255.255 (or in binary form, 11111111 11111111 11111111 11111111), which you add when defining individual IP addresses, specifies that all four octets in the IP address identify the network and host as if there were no subnet mask. In practice, this means that null uses the exact IP address to target performance tests.