Create sequential numbers in a field using Python in the Field Calculator

1. Create a new short integer field.
2. Set the Parser to Python.
3. Select Show Codeblock.
4. Paste the following into the Pre-Logic Script Code: 

rec=0
def autoIncrement():
global rec
pStart = 1
pInterval = 1
if (rec == 0):
rec = pStart
else:
rec += pInterval
return rec

5. Paste the following code in the smaller box below the Pre-Logic Script Code: 

autoIncrement()

6. Click OK.