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On occasion, I'm using a Verizon Mifi adapter to share an internet connection over my office network (which doesn't have internet).

To do this, I have to share the internet connection on the adaptor, which changes my local ip address to a different subnet, along with anyone utilizing the shared resource, in essence disconnecting anyone using the internet from the office network.

So, the office network operates on subnet 192.168.2.0

After sharing, the client now has an ip on subnet 192.168.137.0

This obviously disables us from talking to the office network after joining the internet connection.

Or so I thought. One of my co-workers pointed out that he can still connect to our server using the UNC,

\\servername\sharename

He cannont ping the server with the server ip. However, when he pings the server with its friendly name, it is able to ping and replies with the MAC address (using ARP cache I'm assuming). So instead of resolving the name to the IP, it's resolving to the MAC address.

However, from what I've read here, he still shouldn't be able to connect to the server.

Any helpful explanation would be much appreciated.

silencedogood
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2 Answers2

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The most plausible explanation here is that communication is happening over IPv6. With most modern operating systems IPv6 is enabled by default.

Unlike IPv4 where each host typically had a single address, in IPv6 it is expected for hosts to have multiple addresses at the same time. When the interface first comes up it will only have a "link local" address. Later other types of addresses may be added but the link local address remains.

Peter Green
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  • This is definitely possible. This statement is false however "Unlike IPv4 where each host typically had a single address" - there's nothing in IPv4 that expects a host to have a single address, or only works that way. – mfinni Apr 14 '22 at 05:27
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    Yes it's possible to have multiple IPv4 addresses, but it's not the norm. – Peter Green Apr 15 '22 at 19:08
  • BTW quite a bit of UDP server software doesn't work correctly on machines with multiple IP addresses because the UDP part of berkerly sockets API was not designed to handle them. There are extensions that fix this issue but they differ between operating systems :( – Peter Green Apr 15 '22 at 19:13
  • It's been a long time since this issue cropped up. Not sure why this question was closed... But thanks anyways! Very interesting, I should have thought of that. – silencedogood Apr 21 '22 at 18:06
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There are two possible explanations.

  1. He is using offline files.
  2. He is not disconnected from the local office Network. You can run ipconfig /all from his computer and verify that.
Abu Zaid
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    3. He's resolving the host by name on IPv6 link local. 4. Both machines still have IPX enabled. – joshudson Apr 12 '22 at 19:03
  • @joshudson's comment is consistent with the symptoms presented in the question (able to ping name, unable to ping IP address), which each of those symptoms rules out one of the options presented in the answer. – Ben Voigt Apr 12 '22 at 21:49
  • @BenVoigt: I think is #2 is not eliminated. He could have multi-homed his network adapter and joined the shared internet connection so that he has two IP addresses on that network adapter. – joshudson Apr 12 '22 at 23:07
  • @joshudson: The fact that he can't ping the IP address he knows for the server indicates that he isn't on the network where that address is valid. – Ben Voigt Apr 13 '22 at 15:10