Same IP for 2 NICs on 1 computer

[dimaki]

Hi,
Is it OK for a single computer with several network cards
to have the same static IP address and subnet mask for
both of these cards. The cards connect the computer to two
separate disjoint networks which would not have duplicate
IP addresses for any of the computers on these networks
except for this one computer. This computer does not act
as a router. Would this kind of a setup be problematic in
any way for the computer in question?

Thank you for replies

 

[Phillip Windell]

Hi,
Is it OK for a single computer with several network cards
to have the same static IP address and subnet mask for
both of these cards.

No. Never.

 

[Phillip Windell]

Not only can you not do that,...even with different IP#s, they are not
supposed to be from the same subnet.

175767 - Expected Behavior of Multiple Adapters on Same Network
http://support.microsoft.com/default.aspx?scid=kb;EN-US;175767

 

[dimaki]

Thank you for your reply.

I want to point out that these two networks are physically
disjoint. Yes, they fall within the same subnet range, but
the computers from one network cannot communicate with
computers from the other network and their IP addresses
would not overlap, except of course for this single
computer which is on both of them. Is this still a problem
for this one computer? Clearly it is not a problem for the
rest of the computers on these 2 networks, since there
doesn't need to be communication between the 2 networks

-----Original Message-----
Not only can you not do that,...even with different IP#s, they are not
supposed to be from the same subnet.

175767 - Expected Behavior of Multiple Adapters on Same Network

http://support.microsoft.com/default.aspx?scid=kb;EN-

US;175767

--

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

Hi,
Is it OK for a single computer with several network cards
to have the same static IP address and subnet mask for
both of these cards. The cards connect the computer to two
separate disjoint networks which would not have duplicate
IP addresses for any of the computers on these networks
except for this one computer. This computer does not act
as a router. Would this kind of a setup be problematic in
any way for the computer in question?

Thank you for replies

.

 

[Ed Horley]

Problem lies for the host computer with connections in both networks. If
there are hosts on both sides of each network segment then how will the
server know which interface to forward the traffic through? Causes some
problems in terms of arp/rarp for the host. Wouldn't recommend trying this.

You can however put a simple router with NAT on one segment and create a
little mini segment between the router and the host server. Then simply
build a one-to-one NAT translation on the router to you new segment with the
mapping as the same IP address as your other segment. You can do this
rather easily, problem is maintaining the NAT translations. You can do it
dynamic but it depends on your needs and what requirement you have for
connectivity.

HTH,
Ed Horley
Microsoft MVP Server-Networking

Thank you for your reply.

I want to point out that these two networks are physically
disjoint. Yes, they fall within the same subnet range, but
the computers from one network cannot communicate with
computers from the other network and their IP addresses
would not overlap, except of course for this single
computer which is on both of them. Is this still a problem
for this one computer? Clearly it is not a problem for the
rest of the computers on these 2 networks, since there
doesn't need to be communication between the 2 networks

-----Original Message-----
Not only can you not do that,...even with different IP#s, they are not
supposed to be from the same subnet.

175767 - Expected Behavior of Multiple Adapters on Same Network

http://support.microsoft.com/default.aspx?scid=kb;EN-

US;175767

--

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

Hi,
Is it OK for a single computer with several network cards
to have the same static IP address and subnet mask for
both of these cards. The cards connect the computer to two
separate disjoint networks which would not have duplicate
IP addresses for any of the computers on these networks
except for this one computer. This computer does not act
as a router. Would this kind of a setup be problematic in
any way for the computer in question?

Thank you for replies

.

 

[Phillip Windell]

Even a NAT device needs it to be two different subnets. It needs to know
which network is "trusted" and which is "untrusted". That would be reallt
rough to do if they are all IP#s from the same subnet on both sides.

You might be able to list the individucal machines in the LAT on the
"trusted" side but that would be a real "kudge" and a nightmare to maintain

example:

192.168.1.5 to 192.168.1.5
192.168.1.5 to 192.168.1.5
192.168.1.5 to 192.168.1.5
192.168.1.5 to 192.168.1.5

Problem lies for the host computer with connections in both networks. If
there are hosts on both sides of each network segment then how will the
server know which interface to forward the traffic through? Causes some
problems in terms of arp/rarp for the host. Wouldn't recommend trying this.

You can however put a simple router with NAT on one segment and create a
little mini segment between the router and the host server. Then simply
build a one-to-one NAT translation on the router to you new segment with the
mapping as the same IP address as your other segment. You can do this
rather easily, problem is maintaining the NAT translations. You can do it
dynamic but it depends on your needs and what requirement you have for
connectivity.

HTH,
Ed Horley
Microsoft MVP Server-Networking

Thank you for your reply.

I want to point out that these two networks are physically
disjoint. Yes, they fall within the same subnet range, but
the computers from one network cannot communicate with
computers from the other network and their IP addresses
would not overlap, except of course for this single
computer which is on both of them. Is this still a problem
for this one computer? Clearly it is not a problem for the
rest of the computers on these 2 networks, since there
doesn't need to be communication between the 2 networks

-----Original Message-----
Not only can you not do that,...even with different IP#s, they are not
supposed to be from the same subnet.

175767 - Expected Behavior of Multiple Adapters on Same Network

http://support.microsoft.com/default.aspx?scid=kb;EN-

US;175767

--

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

Hi,
Is it OK for a single computer with several network cards
to have the same static IP address and subnet mask for
both of these cards. The cards connect the computer to two
separate disjoint networks which would not have duplicate
IP addresses for any of the computers on these networks
except for this one computer. This computer does not act
as a router. Would this kind of a setup be problematic in
any way for the computer in question?

Thank you for replies


.

 

[Phillip Windell]

Sorry, I sent that prematurely while doing my "copy-paste" thing and
miskeyed.

The example of the LAT was:

192.168.1.5 to 192.168.1.5
192.168.1.10 to 192.168.1.10
192.168.1.11 to 192.168.1.11
192.168.1.40 to 192.168.1.40
192.168.1.55 to 192.168.1.55

Notice the "from" and "to" are the same number? This is how you would have
to add individual machines to the LAT.

 

[Phillip Windell]

I want to point out that these two networks are physically
disjoint. Yes, they fall within the same subnet range, but
the computers from one network cannot communicate with
computers from the other network and their IP addresses
would not overlap, except of course for this single
computer which is on both of them. Is this still a problem
for this one computer?

Yes, it is still most definitely a problem.

Clearly it is not a problem for the
rest of the computers on these 2 networks, since there
doesn't need to be communication between the 2 networks

No it is not a problem for them. But it is for the machine with two nics.
There is no "right" way to do what you are doing in the manner, the
technology just doesn't allow it. Since you want the two networks to not
see each other then you *must* make them different subnets. That is the
second purpose of routing between different subnets/networks. The first
reason is broadcast management, the second reason is security. You would
perform the restrictions via ACLs on the router.