sed command not working properly

This is my sample file

cat bipin.txt

Unix is an OS
Unix has its own commmands
Unix is a user friendly OS
Unix is platform independent
Unix is a time sharing OS
the best OS to learn is Unix
Abinitio uses Unix in backend

When i use sed 's/Unix/Linux/' bipin.txt , only the first occurrence of Unix must be replaced with Linux right ??

But for me all the occurences are getting replaced .

Could someone please tell me where am making a mistake , please help me

Try sed '0,/Unix/s//Linux/' bipin.txt

It works on all lines, because no address or range is given.
But in each line only the first match is replaced. For example Unix is Unix would become Linux is Unix . This behavior can be altered with modifiers.