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I'm on a LAN provided by an ISP. My computer has its IP address, Subnet Mask, Default Gateway, DNS Servers(using DHCP).

I've been teaching myself bit of networking. I just wanted to apply the concepts I learned to my LAN.

So, what I did is I've figured out all the IPs on my LAN using subnet mask and pinged all of them using a batch script. I just wanted to see the MAC addresses of all the computers on my LAN. Then I've checked my arp cache by issuing command arp -a. I'm surprised to see the result. It was a huge list with all IP addresses (alive on my LAN) as expected but all had the same MAC address.

enter image description here

How did this happen? What's going on? If all nodes have the same MAC address then how will the communication work?

EDIT:

I still don't get it. Let me clarify what I did.

  1. After applying subnet mask /18 or 255.255.192.0 on my IP I got: 182.3.64.0
  2. Which gives me 16384 IP addresses ranging from 182.3.64.0 to 182.3.127.255
  3. First and last IP addresses cannot be used as they are Network address and Broadcast address respectively.
  4. So, My subnet or local network contains 16382 hosts/computers. Right? In other words these 16382 hosts are behind single router.

I want to see the mac addresses of these hosts. SO, I pinged all of them and the above picture shows the result

My thinking:

Now if I think little bit more about it. 16382 hosts behind a single router means too much of broadcast traffic (ARP broadcast). Which is not desirable. At the same time they cannot be connected to different routers because that would mean they are on different networks. May be they are on different VLAN (I don't know much about VLAN but I just know that it helps reducing the broadcast traffic).

I don't understand what is my problem. Kindly identify what am I getting wrong?

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Please add the output from the terminal. Hide the actual addresses (or some parts of the addresses - like the first 5 digits) –  Ярослав Рахматуллин Sep 1 '13 at 17:29
    
@ЯрославРахматуллин: Sorry, I didn't get you. You mean you need screenshot? –  claws Sep 1 '13 at 17:29
    
This might be interesting reading http://serverfault.com/questions/49765/how-does-ipv4-subnetting-work –  Hennes Sep 1 '13 at 18:24
    
@Hennes: I've edited question. –  claws Sep 1 '13 at 19:09
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2 Answers

You can only see the MAC of devices which are on your local network.

If you ping a device behind a router then the communication will flow via that router and you will see that routers MAC. Which means that the single MAC you are seeing is very probably from your router or at least from your default gateway.

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which would suggest that the computers aren't on the same subnet? –  Ярослав Рахматуллин Sep 1 '13 at 17:31
    
Wait a minute! All the IP addresses that we get from subnet. don't they all belong to same local network? –  claws Sep 1 '13 at 17:32
    
Not nessecary. Imagine a few servers with IPs 10.20.30.1 and 10.20.30.2, both connected to a router with two network cards. One card has IP 10.20.30.3 and it connected to a switch with both servers. One has IP 10.20.30.101 and is connected to a switch with your workstation and a printer on it. If you ping the printer you will see its MAC. If you ping either of the servers you will get the MAC of the router. –  Hennes Sep 1 '13 at 17:42
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The list was an overview over which interface could be used to access the various hosts. In this case all interfaces could be reached via the MAC address of your local NIC. Hence one MAC address.

--- Actually that makes no sense. It must be the gateway as Hennes said.

To answer your question, the address you see can only be reached via a gateway and not directly from your local NIC. Any information sent to the those IP addresses go via the gateway's NIC and hence one physical MAC.

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