Interrupt handling - Linux

In linux interrupt programming, we do request_irq(...) in this function, the first argument is irq number. If i am not wrong, this is the interrupt line that we are requesting from kernel. For one particular hardware, is this IRQ line fixed or can it register on any line based on the availability? The concept is not clear. Kindly explain. Also, when i do interrupt programming for AVR or ARM, all the peripherals are having fixed IRQ numbers. and they are having handlers. There is no concept of interrupt lines as such. So my second question is how are IRQ lines and IRQ numbers related? Thanks, Sandeep