IP Address

One of the most important design decisions of an Internet engineer is the assignment of IP addresses, 32-bit numbers that identify Internet hosts. These numbers are placed in the IP packet header and are used to route packets to their destination. Several things should be kept in mind about IP address assignment:

• 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.

• Dynamic addressing. Many hosts, particularly user workstations, do not need to be assigned any particular IP address, and can be dynamically addressed. Aside from obeying the general principles of prefix-based addressing (i.e, hosts on a particular Ethernet must have addresses with a particular prefix), such hosts will operate using any IP address. 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. Dynamically addressed hosts use management protocols to obtain their IP addresses from a server; most statically addressed hosts are manually configured with their IP address.

In addition to the Encyclopedia's resources, Chuck Semeria has written an excellent paper on Understanding IP Addressing.

• Address Classes
• Subnetting
• Classless Inter-Domain Routing CIDR