The functionality for source IP address selection in Windows Server 2008 and in Windows Vista differs from the corresponding functionality in earlier versions of Windows


Windows Server 2008 and Windows Vista have a new TCP/IP stack. When a single network interface has more than one IP address, the new stack selects the unicast address to use as the source IP address. This address selection functionality differs from the corresponding functionality in earlier versions of Windows.

Note Customers most frequently encounter problems when the following conditions are true:
  • When they are using Web servers that have many IP addresses
  • When there are firewall filters that examine the source IP address

More Information

Windows 2008 and Windows Vista follow RFC 3484 for IP V6 source address selection. Because this RFC is written primarily for IPV6, not all the rules in this RFC can be directly applied to IP V4 source address selection. The following rules are followed for IPV4 source address selection only when the application does not specify the source IP address. These rules apply for a particular destination address. If either the destination IP address or the default gateway changes, the source IP address may also change.
  • Prefer same address: If the destination IP address is the same as one of the source IP addresses, use that same address.
  • Prefer outgoing interface: Prefer an IP address on the interface that sends the packet.
  • Use longest matching prefix with the next hop IP address: Use a source IP address together with the longest high order bit match to the next hop IP address.
  • Use longest matching prefix with the destination IP address: Use source IP with longest high order bit match to destination IP address.
Note For the high order bit match rules, convert the source address, next hop address, or destination address to binary. Then, count the number of matching bits from left to right until you come to the first bit that does not match. This gives you the number of matching high order bits.

For more information about IP addressing, visit the following Microsoft Web site:For more information about RFC 3484, visit the following Network Working Group Web site:


This behavior is by design.