prefix length

The number after the slash, and the only thing that says where a network stops. An IPv4 address is a 32-bit number; the prefix counts those bits which, from the left, name the network, the rest naming a host inside it. A /24 cuts on an octet boundary and reads without arithmetic; a /26 leaves two network bits in the fourth octet, so 10.0.0.70/26 belongs to the 10.0.0.64 network, broadcasts at 10.0.0.127 and offers only 62 usable addresses — 2⁶ less the network one and the broadcast one. The same address under two prefixes is on two different networks: an address written without its prefix is an incomplete statement.