Why unidirectional ping in LAN?

[Chris Shearer Cooper]

My LAN looks something like this, with R1 = normal router and R2 = wireless router :

Internet ----- R ----- Desktop
1 --+
|
Nothing ------ R --+
2 ----- Laptop

The problem I'm trying to figure out, is why the laptop can ping the desktop but not vice-versa.

I'm imagining the ping from the laptop hits R2 who notices the destination address is inside the subnet, so he repeats that packet on all his LAN ports, one of which is R1. R1 sees the ping, notices the destination address is inside the subnet, so he repeats the packet on all his LAN ports, one of which is the desktop.

So why doesn't it work the other direction? Is the fact that the laptop is connecting wirelessly relevant somehow?

Thanks!
Chris

 

[Steve Winograd [MVP]]

My LAN looks something like this, with R1 = normal router and R2 = wireless router :

Internet ----- R ----- Desktop
1 --+
|
Nothing ------ R --+
2 ----- Laptop

The problem I'm trying to figure out, is why the laptop can ping the desktop but not vice-versa.

I'm imagining the ping from the laptop hits R2 who notices the destination address is inside the subnet, so he repeats that packet on all his LAN ports, one of which is R1. R1 sees the ping, notices the destination address is inside the subnet, so he repeats the packet on all his LAN ports, one of which is the desktop.

So why doesn't it work the other direction? Is the fact that the laptop is connecting wirelessly relevant somehow?

Thanks!
Chris

You don't say how you've connected the routers to each other. I'm
assuming that you've connected the WAN (Internet) port of R2 to a LAN
port of R1.

In that configuration, R2 blocks pings from the desktop to the laptop.
From the router's point of view, the desktop is part of the Internet,
not part of the local area network, so it drops the ping request.

The laptop can ping the desktop because R2's WAN (Internet) interface
has an IP address in the same LAN as the desktop.

If you want the desktop and laptop to be in the same LAN, use R2 as a
wireless access point only, bypassing its routing capabilities:

1. Disable its DHCP server.

2. Assign its LAN interface an IP address in the same subnet that R1
uses.

3. Connect a LAN port of R2 to a LAN port of R1.

4. Don't connect anything to the WAN (Internet) port of R2.

or:

Remove R1 from the network and just use R2.
--
Best Wishes,
Steve Winograd, MS-MVP (Windows Networking)

Please post any reply as a follow-up message in the news group
for everyone to see. I'm sorry, but I don't answer questions
addressed directly to me in E-mail or news groups.

Microsoft Most Valuable Professional Program
http://mvp.support.microsoft.com

 

[Chuck]

My LAN looks something like this, with R1 = normal router and R2 = wireless router :

Internet ----- R ----- Desktop
1 --+
|
Nothing ------ R --+
2 ----- Laptop

The problem I'm trying to figure out, is why the laptop can ping the desktop but not vice-versa.

I'm imagining the ping from the laptop hits R2 who notices the destination address is inside the subnet, so he repeats that packet on all his LAN ports, one of which is R1. R1 sees the ping, notices the destination address is inside the subnet, so he repeats the packet on all his LAN ports, one of which is the desktop.

So why doesn't it work the other direction? Is the fact that the laptop is connecting wirelessly relevant somehow?

Thanks!
Chris

Chris,

Is R2 running as a WAP right now? With both Desktop and Laptop on the same
subnet?

If that's true, then Desktop and Laptop are connected thru a pair of hubs /
switches. Switch functionality should direct the ping packet to the correct
port. There always could be a problematic switch / WAP component that's
defective, but just because Laptop is connected wirelessly, that's not a normal
possibility.

Try moving each computer to the other router, just temporarily. Make sure that
there's no problem with the router port / router. Try a new patch cable for
Desktop.

Other than a problem with one of the routers (in the switch / LAN side), I'd be
looking at a misconfigured / overlooked personal firewall:
<http://nitecruzr.blogspot.com/2005/05/troubleshooting-network-neighborhood.html#Security>

And look at corrupted LSP / Winsock:
<http://nitecruzr.blogspot.com/2005/05/problems-with-lsp-winsock-layer-in.html>

 

[Chris Shearer Cooper]

I've got nothing connected to the WAN port of R2 ... I've got a cable
running from a LAN port on R1 to a LAN port on R2.

I can't get rid of R1 'cuz it's got my VPN and firewall.

Thanks!
Chris

 

[Steve Winograd [MVP]]

I've got nothing connected to the WAN port of R2 ... I've got a cable
running from a LAN port on R1 to a LAN port on R2.

I can't get rid of R1 'cuz it's got my VPN and firewall.

Thanks!
Chris

Please answer these questions to help other people understand the
situation:

1. What is the IP address of each router's LAN interface?

2. What is the IP address of each computer?

3. Are you pinging the laptop from the desktop by computer name or by
IP address?

4. What exactly happens when you do it? If there's an error message,
what does it say?

5. Is DHCP enabled on R2's LAN interface?
--
Best Wishes,
Steve Winograd, MS-MVP (Windows Networking)

Please post any reply as a follow-up message in the news group
for everyone to see. I'm sorry, but I don't answer questions
addressed directly to me in E-mail or news groups.

Microsoft Most Valuable Professional Program
http://mvp.support.microsoft.com

 

[Steve Winograd [MVP]]

My LAN looks something like this, with R1 = normal router and R2 = wireless router :

Internet ----- R ----- Desktop
1 --+
|
Nothing ------ R --+
2 ----- Laptop

The problem I'm trying to figure out, is why the laptop can ping the desktop but not vice-versa.

I'm imagining the ping from the laptop hits R2 who notices the destination address is inside the subnet, so he repeats that packet on all his LAN ports, one of which is R1. R1 sees the ping, notices the destination address is inside the subnet, so he repeats the packet on all his LAN ports, one of which is the desktop.

So why doesn't it work the other direction? Is the fact that the laptop is connecting wirelessly relevant somehow?

Thanks!
Chris

One possibility is that a firewall program on the laptop is blocking
pings.

I assume that the routers have built-in switches, not hubs. Hubs
repeat an incoming packet on all of their ports, but switches don't.
Switches can tell which specific port to use for sending a packet.
--
Best Wishes,
Steve Winograd, MS-MVP (Windows Networking)

Please post any reply as a follow-up message in the news group
for everyone to see. I'm sorry, but I don't answer questions
addressed directly to me in E-mail or news groups.

Microsoft Most Valuable Professional Program
http://mvp.support.microsoft.com

 

[Chris Shearer Cooper]

Router 1 is 192.168.3.1 on the LAN side (connects to another router on the
WAN side and eventually to the Internet)
Desktop is 192.168.3.160 (static)
There is one other machine on router 1, 192.168.3.10 (dynamic)

Router 2 is 192.168.3.2 on the LAN side (nothing on the WAN side)
Laptop is 192.168.3.11 (dynamic)

From the desktop, I ping 192.168.3.11 and I get "Request Timed Out".
tracert 192.168.3.11 finds nothing (no IP addresses, just stars).

From the laptop, I ping 192.168.3.160 and it's fine. tracert 192.168.3.160
says it's a direct connection (no intermediate IPs)

DHCP is disabled on router 2.

Thanks!
Chris