Maybe drop a ComboBox on the form, populate its "Items" collection with predetermined IP addresses, having the first address be the one it connects to when the application starts, then double-click the ComboBox to get to its SelectedIndexChanged event and add code to change the driver's IPAddress, similar to this:
Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
Me.ModbusTCPCom1.IPAddress = ComboBox1.SelectedItem
End Sub
You might also add some code to the MainForm Load event:
Private Sub MainForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'Either
Me.ComboBox1.SelectedIndex = 0
'Or
For Each item In Me.ComboBox1.Items
If item.ToString = Me.ModbusTCPCom1.IPAddress Then
Me.ComboBox1.SelectedIndex = Me.ComboBox1.Items.IndexOf(item)
End If
Next
End Sub