Which NIC Will Windows Use?

I have a Windows 2003 Server with two separate NICs and 2 separate IPs (the
NICs do not support teaming). Both connect to the same gateway, but are
actually on different private subnets (don't ask me why it was set up this
way ... probably one was private and one was public at some point in this
server's infancy).

Anyway, I was wondering how Windows determines which NIC to use to send
packets out? Let's say a generic request for a TCP request on port 80 made
by a web service app is made. Which NIC will the OS choose? Will it be the
same all the time (the one with the lowest bind number)? Also, does this
same logic apply to Windows 2000/XP?

--
Doug Thews
Director, Customer Solutions
D&D Consulting Services
----------------
Visit my Tech Blog at:
http://www.ddconsult.com/blogs/illuminati/

"Doug Thews" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Anyway, I was wondering how Windows determines which NIC to use to send
> packets out? Let's say a generic request for a TCP request on port 80
made
> by a web service app is made. Which NIC will the OS choose? Will it be
the
> same all the time (the one with the lowest bind number)? Also, does this
> same logic apply to Windows 2000/XP?

The NIC is chosen on the basis of what's in the routing table. A packet
will go out based on which NIC is closest to the target IP address. This
does mean that you will sometimes see packets sourced at one NIC's IP
address going through another NIC, because that NIC is closer to the target
address.

Alun.
~~~~

"Doug Thews" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I have a Windows 2003 Server with two separate NICs and 2 separate IPs
(the
> NICs do not support teaming). Both connect to the same gateway, but are
> actually on different private subnets (don't ask me why it was set up this
> way ... probably one was private and one was public at some point in this
> server's infancy).

Well in this particular case only one will work properly. The Default
Gateway *must* be in the same subnet as the Interface. So since you say
that they are different subnets but yet the same gateway, that means one has
an incompatible gateway and will not do anything outside of it own subnet.

When things are correctly configured,..it is the routing table that makes
the decision for which interface to use based on routes entered into the
table. The Default Gateway is only for "unspecified routes",...there can bo
only **one**. Other nics must leave theirs blank. You can have only *one*
gateway to an "unknown" location via and "unknown" route. All others must
be "known" destinations via a "known" route. The Internet is always
classified as an "unknown".

--

Phillip Windell [MCP, MVP, CCNA]
www.wandtv.com

OK, I see. I mis-spoke when I said they were on different subnets. They
are in fact on the same 10.x subnet and they both have the same default
gateway.

Let's say NIC1= 10.100.1.8 and NIC2=10.100.1.49, with the DG for
both=10.100.1.1.

Now, there's nothing special set up in the routing table (from my
perspective), so my question is ... If I have an application that opens a
port 80 connection to a web service (like Amazon's web services ... on a
206.x.x.x address), how does Windows determine which NIC should be used to
send the packets? My guess is that in this case, the card with the lowest
binding number will always get called.

Thanks in advance for your help.

--
Doug Thews
Director, Customer Solutions
D&D Consulting Services
----------------
Visit my Tech Blog at:
http://www.ddconsult.com/blogs/illuminati/



"Phillip Windell" <@.> wrote in message
news:(E-Mail Removed)...
> "Doug Thews" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> > I have a Windows 2003 Server with two separate NICs and 2 separate IPs
> (the
> > NICs do not support teaming). Both connect to the same gateway, but are
> > actually on different private subnets (don't ask me why it was set up
this
> > way ... probably one was private and one was public at some point in
this
> > server's infancy).
>
> Well in this particular case only one will work properly. The Default
> Gateway *must* be in the same subnet as the Interface. So since you say
> that they are different subnets but yet the same gateway, that means one
has
> an incompatible gateway and will not do anything outside of it own subnet.
>
> When things are correctly configured,..it is the routing table that makes
> the decision for which interface to use based on routes entered into the
> table. The Default Gateway is only for "unspecified routes",...there can
bo
> only **one**. Other nics must leave theirs blank. You can have only *one*
> gateway to an "unknown" location via and "unknown" route. All others must
> be "known" destinations via a "known" route. The Internet is always
> classified as an "unknown".
>
> --
>
> Phillip Windell [MCP, MVP, CCNA]
> www.wandtv.com
>
>

Despite what the settings on a NIC may seem to indicate, you can have
only one default gateway per machine, not one per interface. If you
configure more than one, performance will be unpredictable. RRAS in
particular does odd things when multiple gateways are configured.

In your case, I would just disable one NIC. Having two won't give you
any advantages and may cause you grief.

"Doug Thews" <(E-Mail Removed)> wrote in message
news:#(E-Mail Removed)...
> OK, I see. I mis-spoke when I said they were on different subnets. They
> are in fact on the same 10.x subnet and they both have the same default
> gateway.
>
> Let's say NIC1= 10.100.1.8 and NIC2=10.100.1.49, with the DG for
> both=10.100.1.1.
>
> Now, there's nothing special set up in the routing table (from my
> perspective), so my question is ... If I have an application that opens a
> port 80 connection to a web service (like Amazon's web services ... on a
> 206.x.x.x address), how does Windows determine which NIC should be used to
> send the packets? My guess is that in this case, the card with the lowest
> binding number will always get called.
>
> Thanks in advance for your help.
>
> --
> Doug Thews
> Director, Customer Solutions
> D&D Consulting Services
> ----------------
> Visit my Tech Blog at:
> http://www.ddconsult.com/blogs/illuminati/
>
>
>
> "Phillip Windell" <@.> wrote in message
> news:(E-Mail Removed)...
> > "Doug Thews" <(E-Mail Removed)> wrote in message
> > news:(E-Mail Removed)...
> > > I have a Windows 2003 Server with two separate NICs and 2 separate IPs
> > (the
> > > NICs do not support teaming). Both connect to the same gateway, but
are
> > > actually on different private subnets (don't ask me why it was set up
> this
> > > way ... probably one was private and one was public at some point in
> this
> > > server's infancy).
> >
> > Well in this particular case only one will work properly. The Default
> > Gateway *must* be in the same subnet as the Interface. So since you say
> > that they are different subnets but yet the same gateway, that means one
> has
> > an incompatible gateway and will not do anything outside of it own
subnet.
> >
> > When things are correctly configured,..it is the routing table that
makes
> > the decision for which interface to use based on routes entered into the
> > table. The Default Gateway is only for "unspecified routes",...there
can
> bo
> > only **one**. Other nics must leave theirs blank. You can have only
*one*
> > gateway to an "unknown" location via and "unknown" route. All others
must
> > be "known" destinations via a "known" route. The Internet is always
> > classified as an "unknown".
> >
> > --
> >
> > Phillip Windell [MCP, MVP, CCNA]
> > www.wandtv.com
> >
> >
>
>

If you truely need two IP#s, then run both IP#s on the same Nic. Get rid of
the second Nic.


--

Phillip Windell [MCP, MVP, CCNA]
www.wandtv.com


"Doug Thews" <(E-Mail Removed)> wrote in message
news:%(E-Mail Removed)...
> OK, I see. I mis-spoke when I said they were on different subnets. They
> are in fact on the same 10.x subnet and they both have the same default
> gateway.
>
> Let's say NIC1= 10.100.1.8 and NIC2=10.100.1.49, with the DG for
> both=10.100.1.1.
>
> Now, there's nothing special set up in the routing table (from my
> perspective), so my question is ... If I have an application that opens a
> port 80 connection to a web service (like Amazon's web services ... on a
> 206.x.x.x address), how does Windows determine which NIC should be used to
> send the packets? My guess is that in this case, the card with the lowest
> binding number will always get called.
>
> Thanks in advance for your help.
>
> --
> Doug Thews
> Director, Customer Solutions
> D&D Consulting Services
> ----------------
> Visit my Tech Blog at:
> http://www.ddconsult.com/blogs/illuminati/
>
>
>
> "Phillip Windell" <@.> wrote in message
> news:(E-Mail Removed)...
> > "Doug Thews" <(E-Mail Removed)> wrote in message
> > news:(E-Mail Removed)...
> > > I have a Windows 2003 Server with two separate NICs and 2 separate IPs
> > (the
> > > NICs do not support teaming). Both connect to the same gateway, but
are
> > > actually on different private subnets (don't ask me why it was set up
> this
> > > way ... probably one was private and one was public at some point in
> this
> > > server's infancy).
> >
> > Well in this particular case only one will work properly. The Default
> > Gateway *must* be in the same subnet as the Interface. So since you say
> > that they are different subnets but yet the same gateway, that means one
> has
> > an incompatible gateway and will not do anything outside of it own
subnet.
> >
> > When things are correctly configured,..it is the routing table that
makes
> > the decision for which interface to use based on routes entered into the
> > table. The Default Gateway is only for "unspecified routes",...there
can
> bo
> > only **one**. Other nics must leave theirs blank. You can have only
*one*
> > gateway to an "unknown" location via and "unknown" route. All others
must
> > be "known" destinations via a "known" route. The Internet is always
> > classified as an "unknown".
> >
> > --
> >
> > Phillip Windell [MCP, MVP, CCNA]
> > www.wandtv.com
> >
> >
>
>

I was planning on doing that. What I was wondering is how does Windows
decide which NIC/IP to use if they're both the same length away from the
destination (and yes, they obviously will have the same DG). My guess since
all I'm reading is about "umpredictability" is that the OS will pick the
NIC/IP with the highest bind # in the registry that matches the criteria
(i.e. closest to the destination).

--
Doug Thews
Director, Customer Solutions
D&D Consulting Services
----------------
Visit my Tech Blog at:
http://www.ddconsult.com/blogs/illuminati/



"Phillip Windell" <@.> wrote in message
news:%(E-Mail Removed)...
> If you truely need two IP#s, then run both IP#s on the same Nic. Get rid
of
> the second Nic.
>
>
> --
>
> Phillip Windell [MCP, MVP, CCNA]
> www.wandtv.com
>
>
> "Doug Thews" <(E-Mail Removed)> wrote in message
> news:%(E-Mail Removed)...
> > OK, I see. I mis-spoke when I said they were on different subnets.
They
> > are in fact on the same 10.x subnet and they both have the same default
> > gateway.
> >
> > Let's say NIC1= 10.100.1.8 and NIC2=10.100.1.49, with the DG for
> > both=10.100.1.1.
> >
> > Now, there's nothing special set up in the routing table (from my
> > perspective), so my question is ... If I have an application that opens
a
> > port 80 connection to a web service (like Amazon's web services ... on a
> > 206.x.x.x address), how does Windows determine which NIC should be used
to
> > send the packets? My guess is that in this case, the card with the
lowest
> > binding number will always get called.
> >
> > Thanks in advance for your help.
> >
> > --
> > Doug Thews
> > Director, Customer Solutions
> > D&D Consulting Services
> > ----------------
> > Visit my Tech Blog at:
> > http://www.ddconsult.com/blogs/illuminati/
> >
> >
> >
> > "Phillip Windell" <@.> wrote in message
> > news:(E-Mail Removed)...
> > > "Doug Thews" <(E-Mail Removed)> wrote in message
> > > news:(E-Mail Removed)...
> > > > I have a Windows 2003 Server with two separate NICs and 2 separate
IPs
> > > (the
> > > > NICs do not support teaming). Both connect to the same gateway, but
> are
> > > > actually on different private subnets (don't ask me why it was set
up
> > this
> > > > way ... probably one was private and one was public at some point in
> > this
> > > > server's infancy).
> > >
> > > Well in this particular case only one will work properly. The Default
> > > Gateway *must* be in the same subnet as the Interface. So since you
say
> > > that they are different subnets but yet the same gateway, that means
one
> > has
> > > an incompatible gateway and will not do anything outside of it own
> subnet.
> > >
> > > When things are correctly configured,..it is the routing table that
> makes
> > > the decision for which interface to use based on routes entered into
the
> > > table. The Default Gateway is only for "unspecified routes",...there
> can
> > bo
> > > only **one**. Other nics must leave theirs blank. You can have only
> *one*
> > > gateway to an "unknown" location via and "unknown" route. All others
> must
> > > be "known" destinations via a "known" route. The Internet is always
> > > classified as an "unknown".
> > >
> > > --
> > >
> > > Phillip Windell [MCP, MVP, CCNA]
> > > www.wandtv.com
> > >
> > >
> >
> >
>
>

"Doug Thews" <(E-Mail Removed)> wrote in message
news:%(E-Mail Removed)...
> destination (and yes, they obviously will have the same DG). My guess
since
> all I'm reading is about "umpredictability" is that the OS will pick the
> NIC/IP with the highest bind # in the registry that matches the criteria

That's about it. Also you can have only one DFG and therefore any
"undefined routes" will follow the nic associated with the DFG in the
Routing Table.

--

Phillip Windell [MCP, MVP, CCNA]
www.wandtv.com