3.3.4.1 Introduction

A multihomed host has multiple IP addresses, which we may think of as "logical interfaces". These logical interfaces may be associated with one or more physical interfaces, and these physical interfaces may be connected to the same or different networks.

Here are some important cases of multihoming:

• Multiple Logical Networks

  The Internet architects envisioned that each physical network would have a single unique IP network (or subnet) number. However, LAN administrators have sometimes found it useful to violate this assumption, operating a LAN with multiple logical networks per physical connected network.

  If a host connected to such a physical network is configured to handle traffic for each of N different logical networks, then the host will have N logical interfaces. These could share a single physical interface, or might use N physical interfaces to the same network.

• Multiple Logical Hosts

  When a host has multiple IP addresses that all have the same <Network-number> part (and the same <Subnet- number> part, if any), the logical interfaces are known as "logical hosts". These logical interfaces might share a single physical interface or might use separate physical interfaces to the same physical network.

• Simple Multihoming

  In this case, each logical interface is mapped into a separate physical interface and each physical interface is connected to a different physical network. The term "multihoming" was originally applied only to this case, but it is now applied more generally.

  A host with embedded gateway functionality will typically fall into the simple multihoming case. Note, however, that a host may be simply multihomed without containing an embedded gateway, i.e., without forwarding datagrams from one connected network to another.

  This case presents the most difficult routing problems. The choice of interface (i.e., the choice of first-hop network) may significantly affect performance or even reachability of remote parts of the Internet.

Finally, we note another possibility that is NOT multihoming: one logical interface may be bound to multiple physical interfaces, in order to increase the reliability or throughput between directly connected machines by providing alternative physical paths between them. For instance, two systems might be connected by multiple point-to-point links. We call this "link-layer multiplexing". With link-layer multiplexing, the protocols above the link layer are unaware that multiple physical interfaces are present; the link- layer device driver is responsible for multiplexing and routing packets across the physical interfaces.

In the Internet protocol architecture, a transport protocol instance ("entity") has no address of its own, but instead uses a single Internet Protocol (IP) address. This has implications for the IP, transport, and application layers, and for the interfaces between them. In particular, the application software may have to be aware of the multiple IP addresses of a multihomed host; in other cases, the choice can be made within the network software.