Multi-homed

Multihoming is the practice of connecting a host or a computer network to more than one network. This can be done in order to increase reliability or performance, or to reduce cost.

A typical host or end-user network is connected to just one network. In many circumstances, it can be useful to connect a host or network to multiple networks, in order to increase reliability (if a single link fails, packets can still be routed through the remaining networks), to improve performance (depending on the destination, it may be more efficient to route through one network or the other) and to decrease cost (depending on the destination, it may be cheaper to route through one network or the other).

There are several different ways to perform multihoming.

A single host may be connected to multiple networks. For example, a mobile phone might be simultaneously connected to a WiFi network and a 3G network, and a desktop computer might be connected to both a home network and a VPN. A multihomed host usually is assigned multiple addresses, one per connected network.

In classical multihoming, a network is connected to multiple providers, and uses its own range of addresses (typically from a Provider Independent (PI) range). The network's edge routers communicate with the providers using a dynamic routing protocol, typically BGP, which announces the network's address range to all providers. If one of the links fail, the dynamic routing protocol recognises the failure within seconds or minutes, and reconfigures its routing tables to use the remaining links, transparently to the hosts.

Classical multihoming is costly, since it requires the use of address space that is accepted by all providers, a public Autonomous System (AS) number, and a dynamic routing protocol. Since multihomed address space cannot be aggregated, it causes growth of the global routing table.

In this approach, the network is connected to multiple providers, and assigned multiple address ranges, one for each provider. Hosts are assigned multiple addresses, one for each provider.

Multihoming with multiple addresses is cheaper than classical multihoming, and can be used without any cooperation from the providers (e.g. in a home network) but requires additional technology in order to perform routing:

...
Wikipedia