when we install linux operating system. for swap we give double space than the ram and remaining space for the root partition but instead of this two we also give 100mb space for /boot. is it required?

Short answer, no it is not required.

You can partition Linux in a few ways and having a seperate /boot is one way but many people, I'd think most actually, simple have a /, SWAP, and /home.

Hope that is of some help.

P.S. Welcome to LQ, hope your stay here is fruitful and enjoyable.

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it's required slightly less these days, but still very useful to have a dedicated real partition to pick up the raw guts of the system on boot. It's where the kernel images and bootloader config are called and there. 100mb is just a fairly arbitrary small amount of space. You can put 3 or 4 different kernels in there fairly comfortably without wasting space. Also it should be noted that *technically* the /boot partition is not part of the distributions, and multiple distros should conventionally all put their own kernel images in the same place, with /boot never actually even being mounted by the main system, leaving it untouched during the vast vast majority of runtime and hardly ever written to, meaning that it's extremely unlikely to ever be corrupted etc.

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/boot might be very important if your computer can't boot to a large disk. It is a small amount of room and would help in updated disks in the future. I'd put a bit more than 100M maybe 150M but it is still trivial.

Using /boot can also be necessary if you want a "fancy" filing system on the root partition, as grub doesn't like some of them. It used to be needed if you used logical volume management, as /boot had to be a conventional partition, but I don't know if that's still the case: I've always avoided LVM.

When you do create /boot, you have to be generous or you'll run into trouble if a new version of the boot loader suddenly wants more space. Fedora currently suggests 250MB, not 100.