> Does it means all the network routers running custom linux have to provide sources of entire modification?
Yes.
> I assume if we want companies to make use of linux on a wide level, there should be the ways to overcome this problem(if exists)
What problem? Making the source code of the programs you distribute to your clients available is not a problem - it's an obligation you agreed with when you used the GPL'ed program as a basis for your own.
Yes.
> I assume if we want companies to make use of linux on a wide level, there should be the ways to overcome this problem(if exists)
What problem? Making the source code of the programs you distribute to your clients available is not a problem - it's an obligation you agreed with when you used the GPL'ed program as a basis for your own.
You could do like Sony, who used *BSD on the PS4.