Interfaces on the PIX 525 are numbered top to bottom and left to right (if you have a 4FE card installed) starting at 0. So the top interface should be Gb0 and the second one should be Gb1. The built-in ethernet interface on the right is labeled as ethernet0, the other built-in ethernet interface is ethernet1. Any other ethernet net cards in the PCI slots will start with ethernet 2 and continue up.
Hope this helps.
Scott
