How do i remove the last 5 characters in a string?

Use this

  string a = "192.168.0.225:5010";
  int b = a.Length;
  string c = a.Substring(0, b-5); // change this

  MessageBox.Show(c);

You problems is the misunderstanding and usage of the parameters of Substring

The first parameter

The zero-based starting character position of a substring in this instance.

The second parameter is

The number of characters in the substring.

You would need to supply the Length of the string, minus the size that needed to be removed

string a = "192.168.0.225:5010";
var result = a.Substring(0, a.length - 5);

// or

var result = a.Substring(a.length - 5);

_{Note : This is a one way street to fail-town if the port size differs or is not available}

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Another slightly more robust way would be to Split by the port separator

Splits a string into substrings that are based on the characters in the separator array.

var result = a?.Split(':')[0]

_{Note : This will work even if no port is supplied and return null on a null string, and the string if no port is supplied}

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Or, if you are using .Net Core 3+ you could take advantage of IPEndPoint.Parse then call Address

Converts an IP network endpoint (address and port) represented as a string to an IPEndPoint instance.

var result = IPEndPoint.Parse(a).Address

_{Note : To add even more fault-tolerance you could use the IPEndPoint.TryParse method}