odd sub net behaviour

[bof]

Anyone know of a fix for the following subnet behaviour in Windows XP
Pro?

I have a gateway router on 192.168.1.1 and a PC on 192.168.1.157 with a
subnet mask of 255.255.255.128.

Initially when I configure the .128 subnet mask I cannot ping to
external addresses, which is as I'd expect.

If I subsequently reboot the PC, or if I disable and re-enable the NIC I
can now ping external IP addresses through the router, which I don't
expect to be able to do, and seems wrong that I can access the router on
192.168.1.1 when it's on a different subnet.

The issue seems to be that following a reboot or enable/disable of the
NIC a route is added to the routing table (see below).

So in summary; initially after changing to a .128 subnet routing is as
expected, after a PC reboot routing appears to behave incorrectly. The
behaviour is the same on two independent installs of XP Pro, on
different hardware platforms with two different NICs.


Following is the route print /immediately after/ changing the net mask
to .128, when I can't ping external IPs as expected:


Active Routes:
Network Destination Netmask Gateway Interface
Metric
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
192.168.1.128 255.255.255.128 192.168.1.157 192.168.1.157 20
192.168.1.157 255.255.255.255 127.0.0.1 127.0.0.1 20
192.168.1.255 255.255.255.255 192.168.1.157 192.168.1.157 20
224.0.0.0 240.0.0.0 192.168.1.157 192.168.1.157 20
255.255.255.255 255.255.255.255 192.168.1.157 192.168.1.157 1
255.255.255.255 255.255.255.255 192.168.1.157 3 1
=========================================================================
==
Persistent Routes:
None





Following is the route print /after/ disabling and then re-enabling the
NIC (no other change), when I can ping external IPs, where the PC
appears happy to route via .157 / .1 :


Active Routes:
Network Destination Netmask Gateway Interface
Metric
0.0.0.0 0.0.0.0 192.168.1.1 192.168.1.157 20
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
192.168.1.128 255.255.255.128 192.168.1.157 192.168.1.157 20
192.168.1.157 255.255.255.255 127.0.0.1 127.0.0.1 20
192.168.1.255 255.255.255.255 192.168.1.157 192.168.1.157 20
224.0.0.0 240.0.0.0 192.168.1.157 192.168.1.157 20
255.255.255.255 255.255.255.255 192.168.1.157 192.168.1.157 1
255.255.255.255 255.255.255.255 192.168.1.157 3 1
Default Gateway: 192.168.1.1
=========================================================================
==
Persistent Routes:
None





Apologies if you've already seem this in the .general newsgroup, I'd not
spotted this group when I initially posted.

 

[AJR]

At first glance no "fix" is necessary - it appears XP working correctly. You
are trying to subnet one PC and one router?
The router normally provides IP assignments - unless you are using static
assignments?
When you change the configuration the router function is removed - in this
case not only can you not ping "external" addresses but computers connected
to the router (same network) would not be able to communicate ( no access to
192.168.1.1).
Upon reboot, (changing NICs should have no effect) the router is "reset" to
provide "gateway" function.

 

[bof]

At first glance no "fix" is necessary - it appears XP working correctly. You
are trying to subnet one PC and one router?

No it's a number of PCs, I was just giving an example of how it affects
one particular PC. The effect is consistent across different PCs/XP
installs/NICs.

The router normally provides IP assignments - unless you are using static
assignments?

Apologies for omitting this, yes it's a static IP setup, no DHCP.

When you change the configuration the router function is removed - in this
case not only can you not ping "external" addresses but computers connected
to the router (same network) would not be able to communicate ( no access to
192.168.1.1).

This is the issue, I /can/ ping/route through the router on 192.168.1.1
from a PC on 192.168.1.157 with a subnet mask of 255.255.255.128, but
only after the PC has been rebooted or the NIC has been
disabled/re-enabled. Before reboot or disable/enable it behaves as
expected.

For some reason rebooting the PC or enabling disabling the NIC adds an
extra (unexpected) route to the routing table (see example routing
tables below)

 

[bof]

It sounds like you already know what you're doing but maybe don't see the
difference.
The 0.0.0.0 is a default route added by a routing protocol telling the
machine the way to get to 192.168.157 is via 192.168.1.1. RIP can do this.

Thanks for the reply, whilst I'm not familiar with RIP, I think I
understand what you're saying.

Based on your comments I tried another test, set up the .157 machine
with a .128 mask, as expected no route to 0.0.0.0 through 192.168.1.1.

Disconnecting and reconnecting the Ethernet cable to the NIC caused the
0.0.0.0 0.0.0.0 192.168.1.1 192.168.1.157 20
route to appear in XP's routing table.

Monitoring the traffic immediately after reconnecting Ethernet to the
..157 machine, using Ethereal, there was no RIP traffic, there were SSDP
and IGMP messages for multicast addresses.

The only oddity was the second packet from the host 192.168.1.157
255.255.255.128 (the first packet was a gratuitous ARP to itself), which
was an ARP for 'who has 192.168.1.1? Reply to 192.168.1.157', which I
wouldn't have expected to see as 192.168.1.1 is on another subnet.