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IP Addressing

The most challenging aspect of IP is the construction of the 32-bit IP Addresses.

• Prefix-based addressing. A basic concept of IP addressing is that initial prefixes of the IP address can be used for generalized routing decisions. For example, the first 16 bits of an address might identify Acme Company, the first 20 bits identify the Acme's Jonestown office, the first 26 bits identify a particular Ethernet in that office, and the entire 32 bits identify a particular host on that Ethernet. Prefix-based addressing has its origins in IP Address Classes, and has evolved into Subnetting and CIDR.

• Per-interface assignment. IP addresses are assigned on a per-interface basis, so a host might possess several IP addresses if it has several interfaces. For example, a host with both Ethernet and serial interfaces would have an IP address for each. This is an important consequence of prefix-based addressing. An IP address doesn't really refer to a host, it refers to an interface.

  If a host is known by multiple addresses, then every service on this host can be referred to by multiple names! Addressing this host requires picking one of these. 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.

• Virtual addresses. It is sometimes useful to configure a host with additional IP addresses, usually to use one server to offer Web service for several addresses. Although no real standard exists for such virtual addresses, the typical configuration is to assign additional addresses on an Ethernet (or other multicast medium) and then answer ARP requests for the virtual addresses. Special support is needed both in the server's operating system and in the actual server software.