The noun 'default' in English means a want or absence of something.

[In default of payment for these goods, we should like them returned as soon as possible.]

to mean: If you are not going to pay for these goods, please return them at once.

From this basal meaning it was a short distance for 'default' to acquire the sense of [if nothing else happens].

[In default of my meeting a better girlfriend before I reach twenty-one, I shall marry Sylvia.]

to mean: If I don't meet anybody better, I shall settle for Sylvia.

This is the sense which 'default' usually takes in Computer Science:

[The default operating system for PC's is Windows Vista - though most are able to run Debian or other Linux clones.]

to mean: PC's run on Windows Vista - unless someone has adapted them to run a different operating system.

[The default bootfiles for this system are to be found on the hard disk, but boot can be redirected at the startup prompt to reference the CD device.]

to mean: This computer will boot from its hard disk, unless someone tells it different.

---- I do not know of any case in Computer Science where 'default' takes the meaning 'different'.

There is no such thing as a 'default network'. Perhaps you meant 'default gateway'?

computer science

a default gateway is the default access point a computer or device uses to connect to the internet.

You don't get a degree with a subject; you get a degree in a subject. Bachelor of Science in Computer Science or Bachelor of Computer Science are common descriptions of this degree.