IP connectivity between hosts with differing subnet masks?

Question

For instance, if switch A has hosts with mask /25 and switch B has hosts with /26. Will a ping from Host A to host B be successful?

How does the situation change if:

1. Both switches are connected to a router and a default gateway is provided to all hosts.

2. The subnets are of different classes altogether (/10 and /25)

Answer 1

For instance, if switch A has hosts with mask /25 and switch B has hosts with /26. Will a ping from Host A to host B be successful?

That depends.

Normally each host will check the desintation address for it's packets against the "network" defined by it's IP and mask. If they are within that network they get sent "directly", otherwise they do to the default gateway.

So if host A is within what host B thinks is it's network and host B is within what host A thinks is it's network the ping will succeed.

Further complicating matters some operating systems will try and send packets directly if no default gateway is configured or the default gateway is set to the clients own IP.

IF a router is introduced and packets are sent to it it may router the packets between the hosts. If the router knowns the hosts are on the same network it may may also issue ICMP redirects to tell the clients they can talk directly.

Answer 2

Answer to your original question:

If two switches are a part of different subnets, will they exchange messages if directly connected?

Switches don't look beyond the layer-2 frame to the layer-3 addresses to make decisions on where to switch frames, so the switches play no part in your scenarios, other than to switch frames to the ports where the destination layer-2 addresses are connected.

What you need to look at is how the hosts are configured. A host will compare its network with the network of the destination address to see if they are the same. If they are in the same network, according to the mask configured on the interface, the host will ARP for the destination layer-2 address, or get it from its ARP cache. If the destination address is on a different network, the host will send the packet to its configured gateway.

---

Answer to your new question (please, do not change your question after you have answers to the original question):

IP connectivity between hosts with differing subnet masks?

If one of the networks is contained within the other network, there are two possible answers:

1. If both host addresses fall within the range of the smaller network, then there will be no problem communicating from either host to the other. A ping from one host to the other will succeed.
2. If one of the host addresses is not within the smaller network, then the host from the larger network can send something to the host in the smaller network, but the host in the smaller network will send anything destined for the host in the larger network to its configured gateway because it knows that it is in another network. By default, a ping from one host to another will fail because the host in the smaller network will send a ping request or ping reply to its configured gateway, but a router may possibly be configured to send an ICMP redirect to tell the host in the smaller network to send it directly to the other host.

If you have hosts in two completely separate networks, any packets destined from one host to the other will be sent to the configured gateway of the sending host.