Subnet mask of 255.255.255.255

I have a subnet mask of 255.255.255.255.( Why is my subnet mask 255.255.255.255?). 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 255.255.255.255 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.)

Android

Posted 2012-04-28T06:12:01.833

Reputation: 55

1You 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 – 2012-04-28T07:33:14.317

Okay. That makes sense. – Android – 2012-04-28T09:57:16.687

Firstly, where in the router does it say this and secondly, it will have a another address in front of like 203.1.1.11 255.255.255.255 Please update you Question with this information. the 255.255.255.255 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 – 2012-04-30T21:02:42.613

Answers

255.255.255.255 is a broadcast address same as 0.0.0.0 if you want two endpoints the subnet mask would be 255.255.255.254 (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 192.168.1.0/31 192.168.2.0/31 <-- this /31 means 255.255.255.254 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.

test

Posted 2012-04-28T06:12:01.833

Reputation: 36

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

Ignacio Vazquez-Abrams

Posted 2012-04-28T06:12:01.833

Reputation: 100 516

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 – 2012-04-28T06:30:22.510

1netstat 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 – 2012-04-28T06:32:54.110

But why is it that you are saying the subnet mask is not being followed.(Sorry if i sound too curious) – Android – 2012-04-28T06:44:51.620

I'm not the one saying it; you're the one using the tool. – Ignacio Vazquez-Abrams – 2012-04-28T06:47:37.430

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 255.255.255.255 in the router. How would i change this configuration. – Android – 2012-04-28T06:54:56.270

You would change it in the router, wherever the router has you specify the subnet mask. – David Schwartz – 2012-04-28T07:31:43.430

Firstly, where in the router does it say this?

Secondly, it will have a another address in front of it like 203.1.1.11 255.255.255.255 Please update your question with this information. The 255.255.255.255 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.

onxx

Posted 2012-04-28T06:12:01.833

Reputation: 398

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 I_A and Machine B has IP address I_b.Now
subnet mask remains 255.255.255.255 for both as shown by doing ipconfig. Now bitwise AND of I_A and 255.255.255.255 gives the Network ID for A, that would be its IP address only, as 255.255.255.255 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.

Pallavi Gupta

Posted 2012-04-28T06:12:01.833

Reputation: 1

It seems this does not answer this question. – Renju Chandran chingath – 2014-10-26T07:18:09.277

If the subnet mask is 255.255.255.255, wouldn't the router be on a seperate subnet too? Will the machine still be able to use it as a gateway between the subnets? – osvein – 2017-01-03T15:21:37.753