You're missing a very important and key feature of the .NET framework generally used for input validation: Regular Expressions. Your scenario can be solved with one line of code (or 0 in ASP.NET) using them. The category of your question is set to ASP.NET, so let's first see how to use it in a Web application.
The first step is to drop a RegularExpressionValidator control next to your textbox. Set its ControlToValidate property to the textbox. Set the ErrorMessage to the message you want to give the user when the value is invalid. Finally, set the ValidationExpression to [0-255].[0-255].[0-255].[0-255]. In your code-behind, you can simply check for Page.IsValid to determine if the input was valid or not for all fields that have validators assigned.
Note that the regular expression simply states the groups of values between brackets. You may want to investigate regular expressions further as they provide a very powerful language for expressing requirements on an input string. You may want to take a look at the www.regexplib.com site, which contains a huge number of ready to use expressions for a number of problems.
The code you sent uses the MsgBox function, which is not available/usable on web applications (it's a WinForms message box that is used to display messages on a client application). So if your scenario is actually a WinForms application, here's the code you'd have to use:
' At the class level, a cached version of the compiled expression
Shared ipExpression As New Regex("[0-255].[0-255].[0-255].[0-255]", RegexOptions.Compiled)
' In your method that performs validation
If Not ipExpression.IsMatch(TextBox1.Text) Then
MsgBox("This is not a valid IP address, Please enter the correct one.")
TextBox1.Text = Nothing
End If
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