Basic IP Routing

I have a M/c with 2 NIC cards and I configure the IP addr as 172.18.10.1/255 and 172.18.10.2/255. Now If I ping 172.18.10.1 from some other m/c in the network, the ping fails. Can you pls explain why ?
Meanwhile workaround is to change the IP addr class of the NIC with IP 172.18.10.2.

the "ping" its echo message,when you send "ping" to someone,the destination host have to know the way back to send you echo.
try to "traceroute" or "tracert" to check the way to the destination host,and you will see where the packet "die".
And what this "/255" mask mean ? may be you try to say full class mask,for class B its 255.255.0.0 so its does not really matter the last 16 Host bits just have to be free IP addres that no one else use in the network.

I am sorry for the typo, here is the modified query..

I have a M/c with 2 NIC cards and I configure the IP addr as 172.18.10.1/24 and 172.18.10.2/24. Now If I ping 172.18.10.1 from some other m/c in the network, the ping fails. Can you pls explain why ?
Meanwhile workaround is to change the IP addr class of the NIC with IP 172.18.10.2.

Also I would like to reply to the post by DarkWatcher.. This is not a particular case; its always reproducible. I want to know if its a design issue and if it is, why is it so ?

If you change the IP addr of the 2nd NIC to 10.10.10.1, the ping works. So the basic question is if two NIC's that belong to the same PC have IP addresses belong to the same subnet, why does the ping fail?

Can you give diagram of the toplogy of this network ? then we can talk more about this problem,becouse basicly you have 2 routs to one host but each one with different host number,now I start thinkin of what you have done.I dont see the point to have 2 addresses in one network,its idea if you route the network with this 2 NIC's to give access to the other network for an example 172.18.10.1/24 and 172.18.11.1/24 but its normal that "ping" fail in your case,I have leurn about that but I am not sure in the explanation "why" I will check my notebooks today for that and I will give you better explanation of the problem i think

Does it reply when you ping the other IP 172.18.10.2 ?

If it does, and your using Windows, I think this issue might be related to "which NIC replies to the ping". When 2 NICs are on the same network, two routes for the same network will exist in the PC's internal routing table. However, only one of the routes is used. This means that for outgoing packets, only one of the NICs is used. So when you ping the IP of the 1st NIC (172.18.10.2), the PC replies using the 1st NIC IP as the source address normally. But when you ping the 2nd NIC (172.18.10.1) the PC still replies using the 1st NIC's IP (172.18.10.2) which the pinger didn't actually ping!!. So the pinger thinks it's receiving replies from an unknown PC, so it simply drops the replies.

Another theory would be "ARP responses from the other NIC", but I don't think it fits. Any way, 2 NICs on a host is typically/usually configured with different subnet addresses, this is some times used when you want to run it as a router.

Thanks for the explanation. Indeed I am trying to use the PC as a router for my testing.