With this short entry I would like to continue a little bit with concept explained in Fibre Channel Addressing post. Remember the 24bit FC Address? Let me put the same small image one more time: 24-bit Port Addressing scheme

As you can notice, the Area ID has 8 Bits. Within 8 Bits we can write down two (hex) numbers, which gives us possibilities to represents values 0 – 255 ( hex: 0x00 – 0xff). But what if our Switch (for example Brocade Director) has more than 255 ports? Now, here Shared Area Addressing is becoming handy.

In a few words, a shared area is an Area ID that exist more than once in a single domain. Keep in mind, each port has to have a unique FC Address, those shared areas are differentiated by their Node Address. In oder words, the Area ID is “borrowing” 1 bit from the Node ID and changing it to 1 ( binary 10000000 = hex 80). This 1 bit is used to address ports 256-383.  If that is changed, it is calculated as Area ID  + Node Address  8  (all in hex) Let me give you few examples:

• Port Address 01a800 – Area ID = 0xa8 (hex) = Port Index 168
• Port Address 01a900 – Area ID = 0xa9 (hex) =  Port Index 169
• Port Address 01ff00 – Area ID = 0xff (hex) = Port Index 255  (last possible Port Index without Shared Area)
• Port Address 018880 = 0x88 + 0x80 – 0x8 (hex) = 136 + 128 – 8 (dec) = Port Index 256
• Port Address 018980 = 0x89 + 0x80 – 0x8 (hex) = 137 + 128 – 8 (dec) = Port Index 257
• Port Address 01a880 = 0xa8 + 0x80 – 0x8 (hex) = 168 + 128 – 8 (dec) = Port Index 288

Now this “equation” is not always right, in some cases it is calculated based on equation Area ID + Node Address + 8 (all in hex)

To be completely honest with you, I cannot understand when we should add 8, and when we should subtract 8. If you understand this, I will really appreciate an explanation for example in “comments” section 🙂