The TCP/IP / Subnet screen presents the status of the connection between the protocol stack and the physical LAN interfaces.
A Subnet is associated with an address and 1) an IOP (Input/Output Process) on older platforms, or 2) a LIF (Logical InterFace) on newer platforms.
The Subnet Mask determines when routing of outgoing messages requires the usage of a default gateway.
The TCP/IP address of 127.0.0.1 is a built-in address used for loop-back testing. This is an address that does not go anywhere but is still valid.