One of the challenges of networks is how to signify who the recipients of packets are. This is called the addressing problem. In a single LAN, addressing is simple. Just number each computer and put that number into the packet as the address. Of course, a network administrator must keep track of who has what addresses so there is no confusion and so packets don't get lost or corrupted (altered) or received by two or more different computers.

There is also a security issue here because it might be possible to lie about your own computer's address and thereby snarf up others' packets, thereby eavesdropping on them. Ethernet technology has already thought of this, and each transceiver has an immutable ROM with a pre-assigned Ethernet address burned into it so they cannot be changed.