Unusual Windows XP TCP/IP behaviour

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s155443362

Hello!

We have a large Ethernet segment of 192.168.88.0/21.
There are two host within the network which must communicate
to each other. Their IPs are 192.168.88.255 and 192.168.95.247.

The 192.168.88.255 is a network device and 192.168.95.247
is a Windows XP machine.

The problem is that Windows XP does not take 192.168.88.255
as a unicast IP. Windows XP even dosen't do a ARP lookup
of 192.168.88.255 and begin to send packets to
FF-FF-FF-FF-FF-FF address.

How could I make WinXP think that the 192.168.88.255 is a
unicast IP?

Windows XP configuration:
IP Address. . . . . . . . . . . . : 192.168.95.247
Subnet Mask . . . . . . . . . . . : 255.255.248.0
 
s155443362 said:
Hello!

We have a large Ethernet segment of 192.168.88.0/21.
There are two host within the network which must communicate
to each other. Their IPs are 192.168.88.255 and 192.168.95.247.

The 192.168.88.255 is a network device and 192.168.95.247
is a Windows XP machine.

The problem is that Windows XP does not take 192.168.88.255
as a unicast IP. Windows XP even dosen't do a ARP lookup
of 192.168.88.255 and begin to send packets to
FF-FF-FF-FF-FF-FF address.

How could I make WinXP think that the 192.168.88.255 is a
unicast IP?

Windows XP configuration:
IP Address. . . . . . . . . . . . : 192.168.95.247
Subnet Mask . . . . . . . . . . . : 255.255.248.0
Click to expand...


Looks like windows is screwing up.

In a normal /24 network, the .255 address is reserved as the broadcast
address.

But in your case, the subnet mask of 255.255.248.0 with the IP address
192.168.95.247 clearly defines the local subnet as 192.168.88.0 thru
192.168.95.255. ( or 192.168.88.0/21, as you say. ) The broadcast address
for this subnet is 192.168.95.255.

It seems windows is getting it's subnet calculation wrong, and thinking the
..88.255 address is a broadcast address.

I'm afraid I don't have an XP box at the moment to try to repro this on.

Easiest solution would be to move the problem device off the .88.255
address. After all, you've got a large enough subnet.

PS: a heck of a big broadcast domain, is it not? Is the network
architecture well thought out?
 
Thank you Ron,

Ron Lowe said:
I'm afraid I don't have an XP box at the moment to try to repro this on.
Click to expand...

It's easily reproduceable: when I apply a 192.168.88.255/255.255.248.0
address on XP, then it can't even ping itself. But if I change the address to
191.168..... everything works fine.
Easiest solution would be to move the problem device off the .88.255
address. After all, you've got a large enough subnet.
Click to expand...

That would be an easiest solution, but we have an IP-plan for the network
and I need strong arguments for planning department to change the IP. They
are using Sun Solaris and they don't have any problems with such IPs.
PS: a heck of a big broadcast domain, is it not? Is the network
architecture well thought out?
Click to expand...

We have a plenty of devices on that net, but the subnet is only used for
management and monitoring, not for data traffic.
 
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