Question: What is null route an IP address?
Answer: If your web host told you that they would null route your IP address. This means that your IP address is no longer accessible from the internet. This also means that anyone trying to visit your website would not be able to reach it. If you have your email address hosted with the IP address being null routed. Any email being sent to you will also bounce and not reach you.
Whatever web hosting product you have associated with the IP address may still be functioning but no one from the outside world will be able to access your site. If you have a shared web hosting account, virtual private server, or a dedicated server and the IP address is the only one that is connected to your account. When your IP address is null routed anything that tries to access that IP address from the internet will not be able to. Your account, the VPS, or dedicated server may be functioning and still running but no one can access it if your IP address has been null routed.
A web hosting provider typically performs a null route of your IP address when they suspect that you are under an attack like a DDoS attack. In simple terms a null route of your IP address is like when the only road leading to your office is blocked off.