for all Internet messaging. Each host on the Internet is assigned a unique 32-bit Inter- net address, called the IP address, which is placed in the IP header and used to route packets to their destinations. IP addresses are assigned on a per-interface basis, SO a host can have several IP addresses if it has several interfaces. (Note that a single inter- face can have multiple addresses, too.) Therefore, an IP address refers to an interface, not to the host. A basic concept of IP addressing is that some of the bits of the IP address can be used for generalized routing decisions because those bits indicate which network (and possibly which subnet) the interface is a member of. Addressing is performed on the basis of network/subnet and host; routing is performed based on the network/subnet portion of the address only. When a packet reaches its target net- work, the host portion of the address is examined for final delivery. Internet Addressing and Address Resolution Because humans have a difficult time remembering strings of numbers and because many IP addresses are assigned dynamically, we use DNS to translate between easy-for-humans-to-remember host names and numeric IP addresses. The following sections discuss addressing and routing schemes, two genera- tions of IP addressing (IPv4 and IPv6), and DNS. Addressing and Routing Schemes To start our discussion on Internet and IP addressing, it is necessary to define the various ways in which information can flow between two or more points on a com- puter network. In the IP world, there are three main approaches: unicast, multi- cast, and anycast. Unicast With unicast, sometimes referred to as single-cast, information is sent from one transmitter to one receiver. In other words, information goes to only one location. A unicast server sends a stream to a single user at a time, and each destination address uniquely identifies a single receiver. Multicast Multicast involves sending information from one transmitter to multiple receivers. From this standpoint, it is rather 