My first post & My first Question !!!!

Mos_cow

Good day every one!
This is my first post in this big big forum.... I'm really new to networking ... and have a tiny small confusing Question, at least to me :

Why two PC s can't communicate directly if they in the same broadcasting domain and are in different subnets!!!

As per my understanding ...
When PC A don't know the MAC of an IP it will sned a broadcast arp, since PC B is in the same broadcast domain it will hear this broadcast frame, and atleast B can reply then A can send directly to B??

And now i'll leave this to a one who can correct me and tell where i really missed an important point ?
I really confused here

elegua

Hi,

Welcome to this forum.

Ok, if those two computers are in different subnet, they are in different networks and that's why they can't communicate between them.

Hope this help.

Mos_cow

elegua wrote:

Hi,

Welcome to this forum.

Ok, if those two computers are in different subnet, they are in different networks and that's why they can't communicate between them.

Hope this help.

I totally agree with you on that .... but my Question is WHY they can't communicate ?

phantasm

To go a little further, if both PC's are in different vlan's and your using a layer 2 switch, they wont communicate without a router to do inter-vlan communication (or a layer 3 switch).

Essendon

Broadcasts will not cross a subnet. One must have a bridge (or a router) set to forward broadcasts across subnets or they just won't cross a subnet. Without a router or a L3 switch, PCs in different subnets will not communicate. Put in a router and all should be good.

Both the PCs are in the same broadcast domain and will hear (but not pay attention to) broadcasts set on each others network but they will not be able to directly talk to each other without going through a router because they are on different subnets.

dtlokee

If a host determines the destination is on another subnet it will not send an arp broadcast for the destination host ip address, it will send an arp broadcast for it's default gateway. That is why the two hosts in the same broadcast domain cannot communicate when they are on 2 different subnets.

Essendon

Broadcasts will not cross a subnet. One must have a bridge (or a router) set to forward broadcasts across subnets or they just won't cross a subnet. Without a router or a L3 switch, PCs in different subnets will not communicate. Put in a router and all should be good.

Both the PCs are in the same broadcast domain and will hear (but not pay attention to) broadcasts set on each others network but they will not be able to directly talk to each other without going through a router because they are on different subnets.

Essendon

Broadcasts will not cross a subnet. One must have a bridge (or a router) set to forward broadcasts across subnets or they just won't cross a subnet. Without a router or a L3 switch, PCs in different subnets will not communicate. Put in a router and all should be good.

Both the PCs are in the same broadcast domain and will hear (but not pay attention to) broadcasts set on each others network but they will not be able to directly talk to each other without going through a router because they are on different subnets.

Essendon

OOPSS!!! Sorry I pressed the back button too many times, somehow the same reply got posted again....SORRY

Mos_cow

dtlokee wrote:

If a host determines the destination is on another subnet it will not send an arp broadcast for the destination host ip address, it will send an arp broadcast for it's default gateway. That is why the two hosts in the same broadcast domain cannot communicate when they are on 2 different subnets.

dtlokee special thanks to you .... your statement really cleared the picture....

Also special thanks to everyones help ..... I really appreciate it.

CCIE_2011

take a look at this :


I was experimenting this here are the outcomes:

1- If the vlan 1 interface for two switches are in different ip subnets then you can communicate with other switch normally.
2. for hosts in different vlans:
if you didn't configure the default gateway, the OS will not transmits this packet. If you did configure a default gateway then The OS will communicate directly with the dg if the destination packet is not in the same subnet.



go and try is you'll get shocked .....

To conclude .... your PC is the one who will not allow packets to get out of the NIC if they are destined directly to different subets without passing through you default gateway

Aquabat [banned]

yea the broadcast arp will go to the broadcast address of that subnet, that address will NOT include the other pc because it's in a different subnet.