simple switching question

hi.622823 · ‎10-09-2007

suppose we have 2 computers, pc A and pc B, connected to one switch (both on the same vlan).

the ip on pc A is 192.168.0.1/16, and the ip on pc B is 192.168.5.1/24. will they be able to ping each other? [i said no because pc's act like rudimentary routers which usually know about their connected subnet and a default route. B only knows about 192.168.5.0/24 subnet. a packet from A is on the 192.168.0.0/16 subnet, which won't be in A's route table. however a colleague of mine says this arguement is false. your thoughts are appreciated]

paul.matthews · ‎10-09-2007

They won't. Basically while pcA will think B is in the same subnet, B will need a router to get to A as 192.168.0.1 is outside the range covered by 192.168.1.0/24.

Richard Burts · ‎10-09-2007

I do not believe that there is an absolute answer to this question. If both PCs are in the same VLAN on a single switch then certainly they are in the same broadcast domain and at layer 2 they are clearly able to talk to each other.

I believe that the answer to the question now becomes whether one (or both) of the PCs will ARP for the other one - even though it is not in the local subnet. Some end stations will ARP for remote addresses. If the end stations will ARP for each other (even though they are in different subnets) then yes they will talk to each other. And if they do not ARP for each other then they will not talk.

HTH

Rick

HTH

Rick

hi.622823 · ‎10-09-2007

Rick,

So you're saying the PCs communicate on layer 2 in this scenario. By the way, is there a situation where the PCs would not ARP?

Lastly, lets suppose pc A's IP is 192.168.1.1/24, and pc B's IP is 172.16.1.1/24. Assuming ARP will occur, the PCs should be able ping each other, right?

Richard Burts · ‎10-09-2007

Perhaps we need to be careful about a concept in this discussion. In general ARP is a local subnet (local broadcast domain) function. We expect end stations to ARP for addresses within the local subnet. The question in this discussion is about end stations doing ARP for addresses that are not in the local subnet. There are some end stations (or some configurations of end stations - and perhaps some OSes of end stations) that will ARP for addresses outside of the local subnet. And there are some end stations that will not ARP for addresses outside the local subnet.

In your second question, similar to the first, if pc A and pc B will ARP for each other then they will be able to communicate with each other including the ability to ping each other.

HTH

Rick

HTH

Rick

hi.622823 · ‎10-09-2007

So it boils down to how the OS implements ARP ie how ARP is coded?

I'll have to read the RFC and see what insights it can provide.