In IP protocol version 4, an IP address has 4 bytes. An IP address uniquely addresses a network interface. Such a unique address is called a unicast IP address. If a system uses several network cards (several network interfaces) and all of them use IP protocol, then every network interface has its own IP address. It is similar to the address of a house; every house has only one address.
The other alternative is also possible. Several IP addresses may be assigned on one network interface. The first address is called a primary address while the others are called secondary addresses or aliases. Using secondary IP addresses is common with web servers; for example, the web servers of several different companies, each of which has its own homepage, can run on a single computer/server. The use of secondary IP addresses for web servers is, however, in practice considered as wasteful; so virtual web servers are used instead. In that case, many web servers share one...