An IP address (with IP for Internet Protocol) is an identifier, which is assigned, permanently or temporarily, to each machine connected to a computer network (PC, telephone, smart TV, connected object, ...).
IPv4 (version 4) addresses are 32-bit coded. They are generally represented in decimal notation with four numbers between 0 and 255, separated by dots. Example: 172.16.254.1
A server has as many addresses as there are network cards. Some addresses have a reserved use:
The first bits of the IP address specify the network number, the next bits the host number. The number of bits in the network is specified by the network mask:
When we scan 10.10.10.1/24, we test all addresses from 10.10.10.1 to 10.10.10.255.
192.168.X.X/16, 172.16.0.0/12 and 10.X.X.X/8
The 192.168.X.X/16, 172.16.0.0/12 and 10.X.X.X/8 networks are dedicated to local networks. Such adresses should never be forwarded by routers or boxes to Internet. You must train scans and exploits ONLY on hosts on those networks.