Since the packet is addressed to the interface and not the host, path information is introduced into the address. The exact ramifications of this effect depend heavily on the network design. In particular, careless design can result in a host becoming reachable by one address but not by another. The simplest solution to this problem is to select the host's most reliable interface and advertise its IP address as the host's primary IP address. It is sometimes useful to configure a host with additional IP addresses, usually to use one server to offer Web service for several addresses.
Special support is needed both in the server's operating system and in the actual server software.
Many hosts, particularly user workstations, do not need to be assigned any particular IP address, and can be dynamically addressed. Other hosts, mainly servers, must have a particular IP address because other devices expect them to answer at that address, and must be statically addressed. The address classes differ in size and number. Class A addresses are the largest, but there are few of them. Class Cs are the smallest, but they are numerous.