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The scene:

  • There are two subnets
  • Network A is 192.168.1.0/24 via router 192.168.1.1.
  • There are two hosts, host A1 on 192.168.1.2 and A2 on 192.168.1.3
  • Both A1 and A2 have only one NIC, and none of them are directly on the B network.
  • Both A1 and A2 can see the outside world fine, and can also be reached from outside.

Network B is 10.0.0.0/24, also via router 192.168.1.1 (same as A). A web server B1 is on 10.0.0.2.

The problem:
The web server B1 can be reached by host A2 but not A1. Why is that?

Could it be that the router is missing something or is the problem with host A1?

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Can you post the routing table of the router and 1-1 host from each network ? –  Gabor Vincze Mar 9 '12 at 7:14

2 Answers 2

192.168.1.1 can't be the router for the systems in subnet B.

What is server B1 using as default gateway ?

I assume it is 10.0.0.1 and that ought to be an interface on that same router.

If it works for one host in A and not for the other my first guess is a firewall or access control list getting in the way somewhere.

That could be on the router, on server B1 or even on client A1 itself

Routing table issues seem unlikely as A2 can get through so some routing is working. (It could be that the router has static routes for individual ip-addresses in stead for routes for the whole subnets. It's quite rare to do this, but not impossible so you better check just to make sure.)

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I also thought it seemed odd that 192.168.1.1 is the router for subnet B! But what if the router for B is routing for both subnets A and B? Am I correct that the router must have two IPs, one for each NIC, so 192.168.1.1 (for subnet A) and 10.0.0.1 (for subnet B). Is that all that is needed? I don't have access to the router, so I am only advising. I have a feeling it is either a firewall rule, or perhaps as you say a static route. –  imse Mar 9 '12 at 15:40
    
You are correct. It needs both ip-addresses. And it must be configured to actually route traffic between them. –  Tonny Mar 9 '12 at 22:32

Connectivity checklist:

  1. Can B1 be pinged from A1? Vice-versa?
  2. Examine routing table from A1
  3. Examine routing table from Router 192.168.1.1
  4. Examine routing table from B1
  5. Examine firewall (if any) on B1
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1: Yes 2: 192.168.10.0/24 dev eth0 proto kernel scope link src 192.168.10.2 default via 192.168.10.1 dev eth0 3: don't know (I'm allowed no access) 4. see (3). (5) see (3). –  imse Mar 9 '12 at 15:35

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