SeanVin (17th July 2008)
We had an interesting fault today. Our Anycomms link stopped working. As we have just moved everything onto a new domain, my first thought was DNS, but Anycomms was set up with an IP address. Next I blamed the county end of the link, we always seem get coincidences like this when we change anything significant. However they seemed to think their end is OK. I am inclined to believe them now days, their service and support has vastly improved over the last 18 months.
We are still getting the internet, but out of desperation I try and log onto the router, but I canít. Odd! Next I try to ping the router, I canít do that either, but I am still getting the internet, really odd. I unplug the router and leave it for a minute, plug it back in but still cannot ping it. I dig out an old laptop and connect directly to the router, still canít ping. I Put the laptop onto default sub-net for the router (maybe it has been reset) but still nothing. I dig out the configuration for the router (it was installed before my time) and notice that the subnet mask for the LAN side is 255.255.248.0 (the same as the WAN). This is strange as we use the 10.*.*.* IP range with a subnet mask of 255.0.0.0 however I now have a lead. The Proxy server is obviously working so I check it, this has a sub-net mask of 255.255.0.0 on both ports! I now have a pretty good idea of what has happened but it does not make sense.
When we installed the new domain we set up two new DHCP servers and increased the range of IP addresses that are issued. IT appears that even though the LAN side of the router is on a different subnet to the rest of the network, it can see anything in the 10.0.1.0 to 10.0.7.255 range. Now that weíve changed the DHCP scope, the workstations (including the Anycomms PC) are getting IP address outside this range and so cannot see the router. They can still see the Proxy server and so can get to the internet.
I set the IP address of the laptop to 10.0.0.3 and I could now connest to the router and changed the sub-net mask on the LAN side, everything is now working as it did before.
I always believed that sub-net side of an IP address needed to be identical in order for two hosts to see each other, however it appears that if the least significant bits are set to 0 they are ignored. Is this a bug or a feature?
Its a feature if the address of the contacting pc is inside the address range of the smaller subnet then they can talk fine. The problems occur when an address outside the address range ie 10.50.1.2 attempts to connect to the device as the router sees the machine as outside its subnet and so pays no attention to it.
SeanVin (17th July 2008)
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