# general setting
- given
* host x with IP address ipx and subnet mask MX ⇨ network ID of X = IPX & MX
* destination host Y with address IPV
- if network ID of Y = network ID of X, then X and Y are in the same network; otherwise no
- problem : can't find network ID of Y because we don't know the subnet mask for Y
* note : subnet mask for Y can be different from that of X