Well the default gateway varies for every user.
For example, my default gateway is 192.168.1.1 (that's the router default gateway).

Take a look at this example to get the default gateway:
http://www.java2s.com/Tutorial/CShar...figuration.htm
It's in C# but mentioned and VB.Net are very similar so there shouldn't be any problems porting the code.

You might also want to take a look at this VB.Net example, just to get some ideas (since this code requires user defined information)
http://www.xtremevbtalk.com/archive/.../t-251709.html

Also this VB6 example:
http://bytes.com/topic/visual-basic-...efault-gateway

Happy coding

*EDIT* Negative0: damn, you won this time... it took too long time for me to paste the links xD