How Layer 2 LAN work?

[games130]

Here is the setting:
-Computer A ip 172.27.161.22
-Computer B ip 10.84.10.58

Computer A ------ Layer 2 LAN ------ Computer B

I want to know is why computer A cannot ping Computer B.
From what i know if the two computer are in the same IP segment they will be able to ping each other. I think it work like this. Computer A send the msg to the switch. If the switch does not know who to send it to, it will broadcast the message. When Computer B receive this broadcast it will reply to Switch to let it know the message is for it. So from this A can send message to B

So now back to the problem. Well i am thinking when A send the message to Switch. switch broadcast and computer B(different ip segment) receive it should be able to tell switch that the message belong to it yeah? so A can actually ping B even though they are in different ip segment.

I am really confuse about this. Can please someone explain to me technically on how the switch would actually handle this problem.

Thank alot.

[dshepsman]

AFAIK, a layer 2 switch cant handle it, but a router or layer 3 switch can.

basically, the router knows what networks exist on each of its ports, so when it recieves a packet that needs to go to a diff network, it checks its routing table and pushes it out accourdingly.
it does it by using encapsulation.
layer 2 switches are not capable of encapsulation, and so wont be able to route to a diff network.

make sense?
(bear in mind i have had no sleep for ages, and been nearly a year since i did my CCNA)

[Bleak]

Layer 2 uses physical MAC addressing as opposed to the logical IP addressing used in layer 3.

When going over a layer 2 network the switch will use the MAC addresses to create a point-to-point connection between the devices using the ARP to find the MAC address of a node from the IP address.

However, even though both computers are on the same physical network, they are on separate logical networks and because of that the ping will not reach computer B. This would require a layer 3 device such as a router to forward the packets between separate logical networks. It can decapsulate the packet to read the layer 3 IP address and the encapsulate it again before sending it to the new network, which is known as NAT (network address translation).

Although this might not be technically sound, that is the best way to put it from what I just learnt in my first CCNA module!

[games130]

Well these are not really the answer i was hopping to get. Mabye i will start one question at a time. So when Computer A send the Ping to computer B will the Switch receive this ping message? if so what does the switch do with this message?

[Bleak]

Well these are not really the answer i was hopping to get. Mabye i will start one question at a time. So when Computer A send the Ping to computer B will the Switch receive this ping message? if so what does the switch do with this message?

The switch does not receive the ping message.

The packet is created but never physically leaves Computer A. When the packet is trying to be sent it can't find the IP address on the same subnet. Layer 3 devices (router) are used to transfer packets between subnetworks, a layer 2 device (switch) will not be able to do this. Computer A then drops the packet before it is actually sent.

To answer your original question "I want to know is why computer A cannot ping Computer B." it is because the computers are on different subnets.

[digit]

Computer 1 =
IP - 192.168.10.1
Subnet mask - 255.255.255.0

Computer 2 =
IP - 192.168.10.2
Subnet mask - 255.255.255.0

Give that a whirl and you should be in business. Without the subnets being the same and indeed as bleak was saying a router involved your not going to be able to communicate with your current addressing.

[games130]

The switch does not receive the ping message.

The packet is created but never physically leaves Computer A. When the packet is trying to be sent it can't find the IP address on the same subnet. Layer 3 devices (router) are used to transfer packets between subnetworks, a layer 2 device (switch) will not be able to do this. Computer A then drops the packet before it is actually sent...

ok so in the bold. How does the Layer 2 switch know that the packet is not in the same subnet? Layer 2 cannot read IP right so how?

[peterb]

Bleak was on the right track in his reply -- although this has nothing to do with NAT!

Ping is an IP 'applcation' and IP is a layer 3 protocol, not understood by the switch. The packet doesn't leave the originating computer because it doesn't know what address to give it.

When the packet is sent, the computer checks to see if the destination computer is on the same subnet as itself. If it is, it sends out the packet which passes transparently through the switch. (I say transparently because in fact the switch will have cached the MAC and IP addresses of all computers onte same subnet earlier. If the pact is not on the ame subnet, it sends it to the defauklt gateway, which will be a router that has one interface in the subnet, and another that eventually leads to the destination computer.

(Not only are you using a different subnet you are using a different class of IP address (10.x.y.z) is a class A address, the other is a class C, although that will only be of interest to routers.)

In the case you describe, the packets to the default gateway will be lost because it doesn't exist.

If you defined the default gateway as the computer you are trying to ping, that would probably work.

You can do layer 2 routing using the MAC address using protocols supported by NETBIOS, and I think some other protocols (IPX) also use routing based on MAC addresses.