Different Network/IP Addresses on the same physical cabling

plettner

There's one thing that's been bugging me for a while. Any help appreciated.

If I have 4 computers on an Ethernet network (no routers, just a hub, etc.), 2 computers have the network address 10.8.x.x and two have the network address 192.168.1.x, why can't these talk to each other? I know that both computers on the 192.1681.1.x network can talk and same on the 10.8.x.x.

Why can't a machine on on the 10.8.x.x. network talk to a computer on the 192.168.1.x network. I have not tried this in a lab environment so I was wondering if someone knows a little more about this (I do not have the equipment to do so). Also, would there be an advantage or disadvantage in setting up soemthing like this?

EdTheLad

Ok, ill explain the concept,
Look at the bigger picture, if all the devices in the world were connected to the same ethernet segment there would be a hugh problem, no data would flow due to constant collisions and broadcast traffic.To avoid this a hierarchial infrastructure in needed, this is where ip comes in.Now its possible to break up the broadcast domains by using ip networks.When a device on one network wants to talk to a device on another network the translating between network addresses is done using a router.So different ip addresses represent different virtual networks that
need a router in between to communicate.
This is the reason why you cant ping between these 2 networks, if you added a router onto the network that was configured with a primary and secondary ip address and set the default gateway on your PCs to the relavent ip address of the router interface you could ping between these networks.
What your trying to do isnt so common as generally an ip segment will have all devices on the same network address.

Danman32

The client looks at the destination address, and compares it to the network the PC is on. If it is in a different network, the PC will send the packet to the default gateway if there is one, otherwise it has to discard the request.

If a PC has a 192.168.1 address, its default subnet mask is 255.255.255.0, which means the PC will consider its network to be 192.168.1.x. If the destination IP is 10.8.x.y, 10.8.x does not equal 192.168.1.x so the PC considers them on separate logical networks, even if they are on the same physical network.

Now when a PC sees the destination IP is on the same logical network, then it will ARP the address to find the MAC address to send the packet, assuming the IP isn't already in the ARP cache.

Lee H

hi

of the 2 replies for this post, which MCSE exam would cover this information, just wondering when i will learn this kind of stuff, like IP routers and gateways, i obviously know the basics about each but what exam content goes into detail

any info would be great

lee h

blargoe

Network+.

Microsoft doesn't go into this in depth enough for someone starting from no knowledge.



In your scenario, given the network parameters you entered into your comptuers, the computers with the 192.168.1.x will only be able to talk to 192.168.1.x because that is the only network they are aware of, their local subnet which is defined as the range of addresses from 192.168.1.0-192.168.1.255. For the computer to reach any host that is not on their local subnet they would have to send the traffic to the host that is defined as the default gateway on the network properties on that computer (this would be the router, if you had one). If this default gateway machine doesn't know about the network you are trying to reach, it would send the request to its default gateway, and so on until the request times out.

For this to work, you'd have to have a router with an interface on the 192.168.1.x network and an interface on the 10.8.x.x network, and set the computers to use the router interfaces as the default gateway.

Lee H

hi

one solution we were looking for regarding a DHCP server including an IP range that was on a different subnet, we had 172.20.244.1 - 172.20.245.254 subnet mask 255.255.254.0, we were running out and were given more IP's but the subnet was different, we were being advised to setup DHCP relay, not sure but i think you need a layer 3 switch. Would this solution work for this scenario aswell

lee h

Danman32

I do recall questions either on 218 or 292 where you had to figure out why a user on a WS couldn't get out of the network.

Also basic routing is certainly covered in 293.

I didn't take 290 or 291 since I used W2K to get MCSA, then used 292 to upgrade to 2003 and continue on MCSE track from there.

plettner

Thanks for the comments, everyone.

Through my Network+ learning, I know that the computers won't see another computer with a different network address. The texts I used never really mentioned why. But maybe I read too much into it. I guess I also take a lot of the "mechanics" of network for granted.

I'm still working on my MCSA and eventually MCSE so there's plenty more to learn.