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
Customers most frequently encounter problems when the following conditions are true:
- When they are using Web servers that have many IP
- When there are firewall filters that examine
the source IP address
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
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.
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:
behavior is by design.