when a node flogis into the fabric the fabric controller assigns a 3-byte S_ID/D_ID. This is in hex and for a Mcdata switch would take the form of 0x7C0D13 (Mcdata always ends in 13). the S_id contains info on the domain id the node has logged into (1st byte 7C) and the port it is logged into (2nd byte 0d), these are at offsets. By using S_Id lockdown you are tying the WWPN of the node to a particular S_Id, so that if someone tried spoofing your WWPN , they would have to plug into a different port and therefore have a different S_ID.