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This totally sounds like a bug to me, because my subnet mask and IP address should be valid by all accounts.

My ISP has assigned me a fixed IP address of 65.110.7.20, with a subnet mask of 255.255.255.252. Which sounds about right, because we're trying to set up a router with a /31 network - just 4 ip addresses.

But when I try to set this IP and netmask up in Windows XP, I get the following error:

"The combination of IP address and subnet mask are invalid. All of the bits in the host address portion of the IP address are set to 0."

But they're not all set to 0. That would be a subnet mask of 255.255.255.255. Even then, that too is a valid subnet mask, consisting of a network of 1 IP address.

Is there a way around this? A registry hack maybe?

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In one place you say 255.255.255.252 in another you say /31. These are two different things. My assumption in my answer is that you meant 255.255.255.252, which is /30. –  MDMarra Jun 9 '10 at 17:07
    
You're right. I was looking at the wrong line on my subnet table. :) –  Ernie Dunbar Jun 9 '10 at 17:10
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2 Answers

The first address in a network is not valid for a host, as it is used to identify the network.

65.110.7.20/30 breaks down like this


65.110.7.20 - Network identifier - not usable for a host
65.110.7.21 - OK to be used by a host
65.110.7.22 - OK to be used by a host
65.110.7.23 - Broadcast address for network - Not usable for a host


In reality, you only have 2 usable addresses. I bet if you set the IP to a valid host address, you will have no issues.

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@ qq subnet-calculator.com disagrees with you there. With the 252 subnet, there are only two host addresses avialable –  Sarge Jun 9 '10 at 17:14
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@Sarge - He edited very quickly after posting that. Or should I saw he edited very quackly :). Good thing you can't downvote a comment, I'd lose a ton of rep for that awful joke –  MDMarra Jun 9 '10 at 17:18
    
@Sarge, typically the high end of the IP range is used for the IP broadcast address. –  heavyd Jun 9 '10 at 17:22
    
@heavyd - I don't see Sarge saying anything to the contrary. Did I miss something? –  MDMarra Jun 9 '10 at 18:01
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From my Subnet Calculator / Planner

65.110.7.20 - 65.110.7.23 CIDR = 30 Mask = 255.255.255.252

The ability to use the 0 address, in this case 65.110.7.20, is implemention dependent. See Cisco Subnet Zero.

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