Detecing Network

If you have an Active Directory in your network, you can retrieve machines with the System.DirectoryServices namespace, Just add a reference System.DirectoryServices.dll, then add the following code to the Form Load Event(you need a multiline TextBox TextBox1 on your Form):

//... Add this to the top of your code:
Imports System.DirectoryServices

//...Add this inside the Form Load EventHandler:

        Dim objDirEnt As DirectoryEntry = New DirectoryEntry(adpath,username,password)
        Dim objChildDE As DirectoryEntry
        For Each objChildDE In objDirEnt.Children
            TextBox1.Text += objChildDE.Name + vbNewLine
        Next objChildDE

The adpath you'll have to figure out for yourself. Try something like this(replace DOMAINNAME):
Dim adpath As String = "LDAP://DOMAINNAME.LOCAL/CN=Computers,DC=DOMAINNAME,DC=LOCAL"

... You can swap "CN=Computers" for "CN=Users" to get all the users.

For username and password, the credentials of a user with administrator rights to the server should be enough.

Dim username As String = "UserWithServerAdministratorPrivileges"
Dim password As String = "UsersPassword"

there is no active directory , it just a simple netwrok