Order by last 2 characters string

Try this:

SELECT port 
FROM interfaces 
ORDER BY SUBSTRING_INDEX(port, '/', 1), CAST(SUBSTRING_INDEX(port, '/', -1) AS SIGNED)

Check the SQL FIDDLE DEMO

OUTPUT

|   PORT |
|--------|
|  Fa0/1 |
|  Fa0/2 |
|  Fa0/3 |
|  Fa0/4 |
|  Fa0/5 |
|  Fa0/6 |
|  Fa0/7 |
|  Fa0/8 |
|  Fa0/9 |
| Fa0/10 |
| Fa0/11 |
| Fa0/12 |
|  Gi0/1 |
|  Gi0/2 |
|  Null0 |
|  Vlan1 |

Why you need + 0 ?? Simply remove it and it will work.

SELECT port FROM interfaces ORDER BY RIGHT(port, 2)

SQL Fiddle Demo

Output:

PORT
------------
Fa0/1
Gi0/1
Gi0/2
Fa0/2
Fa0/3
Fa0/4
Fa0/5
Fa0/6
Fa0/7
Fa0/8
Fa0/9
Fa0/10
Fa0/11
Fa0/12
Null0
Vlan1