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I have a subnet mask of Why is my subnet mask I refer that from the above question that it is as good as each computer is on its own network and cant interact with one another. But if I use armitage (part of metasploit) and scan for hosts on my network, it shows me all the computers connected to my local network.

How is this possible when my subnet mask of forces the computers to be on a different network?

Also is there any way I can change this subnet mask so I can put all the computers join the same network? (I use windows but even if I go to ipv4 properties and set a default subnet mask it does not seem to get reflected in the router.)

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You are confusing the subnet mask on the WAN with the subnet mask on the LAN. The point of a router is to route between two distinct networks and each network has its own subnet mask. Your WAN link is a point to point link, not a broadcast network that bridges a subnet. It links a machine with a single IP address to another machine with a single IP address, each of which has an interface route to the other end of the link. – David Schwartz Apr 28 '12 at 7:33
Okay. That makes sense. – Android Apr 28 '12 at 9:57
Firstly, where in the router does it say this and secondly, it will have a another address in front of like Please update you Question with this information. the refers specificly to the subnet mask and not at IP address. This tells the router how many bits are being used for the specified subnet. – onxx Apr 30 '12 at 21:02
up vote 1 down vote accepted is a broadcast address same as if you want two endpoints the subnet mask would be (however this still won't do what you are asking. What you are asking for is VLANs or PVLANs (virtual or private lans). This would require hardware capable of layer2 switching and VLAN support at least. However you would still need layer3 routing abilities to talk between VLANs. The VLAN will virtualize the collision domain and thus VLAN2 does not see VLAN3 unless defined in a layer3 router connecting both VLAN2 and VLAN3. A poorman's version would be n+1 ethernet ports in a computer acting as a router and network switch where "n" is the number of pcs and the "+1" is the port going out to the internet. The computer would need to run some software like linux as a firewall/proxy/router and define all ports to different subnets like <-- this /31 means which allows exactly two hosts. The reason your scan tool sees everything is how the software works by incrementing the address by one then asking for an echo response at that address and the fact that the subnet mask your are using is also a broadcast address. Presently all your pcs are on the same collision domain so the see each other just fine and ipv4 trickery is not going to separate them.

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The subnet mask is merely a guide for software; routing determines which addresses are valid choices for attempted communication with.

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Where can i view the routing options then. Also i though that it was the subnet mask the identifies the network id and host id thus diifferentiating between computers on a different network. – Android Apr 28 '12 at 6:30
netstat or route can give you your routing table; see your platform's documentation. The subnet mask only identifies things for those that follow it, much like curbs only keep cars off the sidewalk because they usually don't go over them. – Ignacio Vazquez-Abrams Apr 28 '12 at 6:32
But why is it that you are saying the subnet mask is not being followed.(Sorry if i sound too curious) – Android Apr 28 '12 at 6:44
I'm not the one saying it; you're the one using the tool. – Ignacio Vazquez-Abrams Apr 28 '12 at 6:47
Okay i guess i have to clear things up a little. I run a windows 7 system and i have explicitly spefied the subnet mask in the ipv4 properties. Still i see that the subnet mask is in the router. How would i change this configuration. – Android Apr 28 '12 at 6:54

Firstly, where in the router does it say this?

Secondly, it will have a another address in front of it like Please update your question with this information. The refers specifically to the subnet mask and not an IP address. This tells the router how many bits are being used for the specified subnet. Usually only used when a router makes a Public IP connection to the ISP or in similar environments.

Thirdly, please describe your internet/network set-up.

In windows 7 you can use the ipconfig /all to get all your IP address information from the command prompt.

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This is because a host on a network always contacts its router irrespective of any other host in its network.Suppose Machine A has IP address as IA and Machine B has IP address Ib.Now
subnet mask remains for both as shown by doing ipconfig. Now bitwise AND of IA and gives the Network ID for A, that would be its IP address only, as contains only 1's. Similarly Network Id of B will be its own IP address. This will ensure that packet is always sent to router first because Machine A and B always lies in different Networks.

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It seems this does not answer this question. – Renju Chandran chingath Oct 26 '14 at 7:18

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