this might be a stupid question for you brainiacs here but here goes.
i use a usb to serial adaptor to connect to Cisco routers using Putty.
I was alway using the same usb port on my laptop (com4) but suddenly the other day the same usb port became com 3,why would it suddenly change?
isn't like usb1 =com1 and so on?

Did the driver get updated? That might do it.

There is no relationship between COM ports and USB ports. I have two radio programming cables that use the same USB port. One is assigned as COM 3, the other is COM 4.

Yeah, but when you move one of those USB/serial cables to a different usb port, it will become a different com port. You've just associated com-X with USB/Serial cable X and Com-Z with USB/Serial cable Z, while using one particular USB port.

Yep, I started to say, always use the same usb port every time when using a serial/usb adapter.

We've run into this issue with our computers that connect to alarm panels for programming. I mark the port on the PC and tell the user not to move the cable - if they need to switch it, they unplug the 25 pin connector from the respective modem.