Multihoming for IPv4
A version of this page is also available for
4/8/2010
When a computer is configured with more than one IP address, it is referred to as a multihomed system.
Note
Multihoming is the same in IPv4 and in IPv6.
Windows Mobile supports multihoming in the following ways:
- Multiple IP Addresses per NIC
- Multiple NICs per Physical Network
- Multiple Networks and Media Types
Multiple IP Addresses per NIC
NetBIOS over TCP/IP (NetBT) binds to only one IP address per interface card, because only one address is registered when a NetBIOS name registration is sent out. This registration will occur over the IP address that is listed first in the registry.
Multiple NICs per Physical Network
There are no restrictions other than hardware.
Multiple Networks and Media Types
There are no restrictions other than hardware and media support.
When an IP datagram is sent from a multihomed host, the host hands it down to the interface with the best apparent route to the destination. Accordingly, the datagram may contain the source IP address of one interface in the multihomed host, yet be placed on the media by a different interface. The source MAC address on the frame is that of the interface that actually transmits the frame to the media, and the source IP address will be the one that the sending application sourced it from.
When a computer is multihomed with NICs attached to disjoint networks (networks that are separate from and unaware of each other, such as a network connected to using Remote Access Service, also called RAS), routing problems may arise. It is often necessary to set up static routes to remote networks in this situation.
Note
It is important to understand that there can only be one active default gateway for a computer at any moment in time.
When configuring a computer to be multihomed on two disjoint networks, the best practice is to set the default gateway to be the one on the main or largest and least-known network. Then either add static routes or use a routing protocol to provide connectivity to the hosts on the smaller or better-known network. Avoid configuring a different default gateway on each side, as this can result in unpredictable behavior and loss of connectivity.
For more information about name registration and resolution and choice of NIC on outbound datagrams with multihomed machines, see Host Name Resolution for IPv4, and Windows Sockets.
For information about IPv6, see Host Name Resolution for IPv6 and Neighbor Discovery (ND).