Well now thats an interesting question. It could be a variety of layers. You need to understand how SMTP works, basically your mail server will accept the mail, and forward it to another mail server, to another mail server etc etc. If you view the email headers you'll see each one of these hops as a "recieved from... " header.
Anyway, so lets say one of these mail servers along the way cannot contact the next hop mailserver because of layer 3 (IP) connectivity trouble.. then when the mail bounces (after a certain time has expired), the mailserver will return it to you.. then the error would be a network layer error.
However lets say that the SMTP server is able to communicate with the next hop, but for some reason it cannot send the mail (maybe the software is not working, or the mail queue is full etc etc). Then this would be an error at some other layer.. perhaps you could say its an application layer error since SMTP sits at the application layer.. though strictly its not an error that you can corelate to any layer of the OSI model.
Remember the OSI model is just that - a model - its meant to provide a standard for how different systems interconnect, you also have the DARPA model which has less layers but does exactly the same stuff. In other words its a matter of perspective.
However its an interesting question... may I ask what it was relevant to ?
well i am studyin for ccna...n it jus poped into my head...
one more qn on ip adressing
the following five are valid subnet numbers in network 126.96.36.199 when using 255.255.255.0 as mask...
alsoassumin a subnet mask of 255.255.224.0 which 3 of the followin would be valid host addresses?
and what is loop back testing?
thanx a million guys
with regard to your first question,
the different options you've given are all separate subnets.. you have to look at the subnet mask to determine the network ID and the host ID:
in this case the network ID is all the octets with a corresponding 255.. in other words the first three.. so the network ID is 180.1.0 and the host ID will be the last octet. Therefore all the different options you've given are all separate subnets.
I didn't quite get your second question.. but I think your answer would be the fourth one (188.8.131.52) simply because 256-224 = 32 which means that all the subnets have to be multiples of 32.. the fourth option is the only one that meets this criteria.
Your last question is very easy.. loopback is 127.0.0.1 which is the IP address that refers to your computer... if you ping 127.0.0.1, you're basically pinging your own computer.. if this fails, it means the TCP/IP stack is not properly installed on your system.