In Jenkins, how can parameters that contain spaces be referenced?
You can use params['IP Address'].

Think of params as a Map containing a key 'IP Address'. If the key didn't have a space, then you could use params.IPAddress or params['IPAddress'], but when there's a space, you can only use the latter syntax.

That syntax is also useful when you're computing the key name to look up, and have stored it in a variable:

final String myParam = "param${i}"
// can't say `params.myParam`
echo "Value is ${params[myParam]}"