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Question
Wednesday, October 3, 2018 11:05 AM
My IP on the physical network I am on is 192.168.7.89 (interface 22). I also have a VPN connection to x.165.115.y (obscured for security reasons). My IP address on the VPN connection is 192.168.10.2 (interface 49). The route table is as follows:
===========================================================================
Interface List
22...66 3f 51 d8 6c 29 ......Intel(R) Dual Band Wireless-AC 7265
17...34 97 f6 75 69 fb ......Realtek PCIe GBE Family Controller
21...a4 02 b9 71 03 aa ......Microsoft Wi-Fi Direct Virtual Adapter #5
24...a6 02 b9 71 03 a9 ......Microsoft Wi-Fi Direct Virtual Adapter #6
49...........................Seattle (78.x only)
23...a4 02 b9 71 03 ad ......Bluetooth Device (Personal Area Network) #2
1...........................Software Loopback Interface 1
===========================================================================
IPv4 Route Table
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 192.168.7.1 192.168.7.89 2
x.165.115.y 255.255.255.255 192.168.7.1 192.168.7.89 2
127.0.0.0 255.0.0.0 On-link 127.0.0.1 331
127.0.0.1 255.255.255.255 On-link 127.0.0.1 331
127.255.255.255 255.255.255.255 On-link 127.0.0.1 331
192.168.7.0 255.255.255.0 On-link 192.168.7.89 257
192.168.7.89 255.255.255.255 On-link 192.168.7.89 257
192.168.7.255 255.255.255.255 On-link 192.168.7.89 257
192.168.10.0 255.255.255.0 192.168.78.1 192.168.10.2 46
192.168.10.2 255.255.255.255 On-link 192.168.10.2 301
192.168.78.0 255.255.255.0 192.168.78.1 192.168.10.2 245
224.0.0.0 240.0.0.0 On-link 127.0.0.1 331
224.0.0.0 240.0.0.0 On-link 192.168.7.89 257
224.0.0.0 240.0.0.0 On-link 192.168.10.2 301
255.255.255.255 255.255.255.255 On-link 127.0.0.1 331
255.255.255.255 255.255.255.255 On-link 192.168.7.89 257
255.255.255.255 255.255.255.255 On-link 192.168.10.2 301
===========================================================================
The above route table is the default by Windows except the 192.168.78.0 route which I added manually.
The ARP table for that VPN interface always shows empty entries, even immediately after I successfully ping a destination: 192.168.78.83.
I manually added an entry for 192.168.78.83 and the ARP table is as follows:
Interface: 192.168.10.2 0x31
Internet Address Physical Address Type
192.168.78.1 static
192.168.78.83 static
224.0.0.22 static
224.0.0.251 static
239.255.255.250 static
255.255.255.255 static
Why is ARP table empty? I am troubleshooting some intermittent network issues with 192.168.78.83, and I want to eliminate the possibility that it is an ARP problem. How do I find out whether my ARP is working?
All replies (1)
Thursday, October 4, 2018 5:16 AM âś…Answered
MAC addresses don't transit routers. You will never see the MAC address of a host that is not on the same subnet as your host.
Address Resolution Protocol (ARP) is used to resolve an IPv4 address (32 bit Logical Address) to the physical address (48 bit MAC Address). Network Applications at the Application Layer use IPv4 Address to communicate with another device. But at the Datalink layer, the addressing is MAC address (48 bit Physical Address).
If a source device want to communicate with another device, source device checks its Address Resolution Protocol (ARP) cache to find if it already has a resolved MAC Address of the destination device. If it is there, it will use that MAC Address for communication.
If ARP resolution is not there in local cache, the source machine will generate an Address Resolution Protocol (ARP) request message, it puts its own data link layer address as the Sender Hardware Address and its own IPv4 Address as the Sender Protocol Address. It fills the destination IPv4 Address as the Target Protocol Address. The Target Hardware Address will be left blank, since the machine is trying to find that.
The source broadcast the Address Resolution Protocol (ARP) request message to the local network.
The message is received by each device on the LAN since it is a broadcast. Each device compare the Target Protocol Address (IPv4 Address of the machine to which the source is trying to communicate) with its own Protocol Address (IPv4 Address). Those who do not match will drop the packet without any action.
When the targeted device checks the Target Protocol Address, it will find a match and will generate an Address Resolution Protocol (ARP) reply message. It takes the Sender Hardware Address and the Sender Protocol Address fields from the Address Resolution Protocol (ARP) request message and uses these values for the Targeted Hardware Address and Targeted Protocol Address of the reply message.
The destination device will update its Address Resolution Protocol (ARP) cache, since it need to contact the sender machine soon.
Destination device send the Address Resolution Protocol (ARP) reply message and it will NOT be a broadcast, but a unicast in order to save network resources.
The source machine will process the Address Resolution Protocol (ARP) reply from destination, it stores the Sender Hardware Address as the layer 2 address of the destination.
The source machine will update its Address Resolution Protocol (ARP) cache with the Sender Hardware Address and Sender Protocol Address it received from the Address Resolution Protocol (ARP) reply message.
Machines at local network can't communicate if they don't know the MAC Address of each other. Neither Internal IP Address can be used for that. If a router wants to communicate with its client or with the other router then it must know the MAC Address of its client and the other router as well.
Please refer to this similar case for some ideas
Unresolved MAC address in ARP table
https://supportforums.adtran.com/thread/3725
Please Note: Since the website is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
Regards
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